What Kind Of PCB Is Good Quality?


With the rapid development of mobile phone, electronics, telecommunications and other industries continue to grow and also prompted the PCB circuit board industry quantity at the same time, people requirements are more and more high in components of number, weight, color, material, precison and reliability.

However,due to the fierce competition in the market price, the cost of PCB circuit board material is also in the rising trend, more and more manufacturers at a low price to monopolize the market in order to enhance the core competitiveness. However, these ultra low cost behind is reduction of material cost and production cost, lead to the devices are usually easy to crack,scratch, comprehensive factors included the precision, performance were not certificated, the serious influence the solderability and liability of products use.

In the face of the market all kinds of PCB circuit board, a good circuit board can distinguish from two aspects; the first method is to judge from the appearance, the other is from the PCB board specification quality.

Method for judging the quality of PCB circuit board:

First: from the appearance of the circuit board to distinguish between good and bad.

Generally, the appearance of the PCB circuit board can be analyzed and judged in three aspects;

1.Standard rules for size and thickness

Circuit board to standard circuit board thickness is different size, customers can measure and check according to their product thickness and specifications

2.Light and color.

The external circuit board is covered with ink, and the circuit board can play an insulating role. If the board color is not bright and the ink is less, the insulation board itself is not good.

3. Weld appearance.

Because of the more parts of the circuit board, if the welding is not good, the parts will easily fall off the circuit board, seriously affecting the welding quality of the circuit board, the appearance is good, carefully identified, the interface stronger is very important.

Second: high-quality PCB circuit boards need to meet the following requirements

    1.  Copper surface is not easy to oxidize, affect the installation speed, oxidation soon after the bad;
    2. After the components are installed, the telephone is easy to use. That is to say, the electrical connection should meet the requirements;
    3. By the high temperature copper skin is not easy to fall off;
    4. The line width, line thickness, line spacing meet the requirements, so as to avoid line heating, break, and short-circuit;
    5. No additional electromagnetic radiation;
    6. And high temperature, high humidity and resistance to special environment should also be considered within the scope;
    7. The shape is not deformed, so as not to install after the deformation of the shell, screw hole dislocation. Now all mechanical installation, the circuit board hole position and line and design deformation error should be within the allowable range;
    8. The surface mechanical properties should meet the installation requirements;

The above is the PCB circuit board to judge the good and bad methods, in the purchase of PCB circuit board, be sure to polish your eyes.

PCB Assembly Files- PCB Enginner Need To Know


Some complex PCBs design be finished,you must take PCB to assembly plant when PCBs get back from PCB manufacturer.That assembly plant need you provide some necessary documents for production.Where the captain guy gave you talk about the PCB assembly when production of documents required,and what the representative of the file is useful.

PCB assembly processing required documents are the following:

1.Stencil file

Stencil file is solder paste file, it used in brush paste when PCB assembly, it can be automatically welded when PCB through high temperature furnace. But stencil file didn’t be included plug-in components information,it’s surface mount components information.What kind of stencil is? The following is a kind of stencil.

Above aperture is exposed for each surface mount components,no components area are covered. And the stencil is a certain thickness,the exposed holes are filled with solder paste when brush paste, then pcb pad coated with solder paste. The solder paste melting to stabilize assembly when soldering in high temperature.

2.Pick Place file

When assembly is processed with a machine.you have to tell the machine how to paste the components,which requires the coordinates of the document.Coordinate file also call pick place file,it has the coordinates of each components, layer, rotation angle and other information,the machine will be able to read this information,according to the requirements to paste components.

There are components labels.X,Y coordinates,rotation angle,where level.

3.Component bitmap file

The component bit map file,in fact is the component silkscreen. It’s provided to operate workers to comparison in the production processing.Inspect if there’s any wrong of assembly.The component bitmap is shown below:

4.PCB assembly fixture file:

PCB assembly fixture file, it’s used in covering surface mount components and expose plug pin when PCB through reflow welding machine.Because in the production of the first process is surface mount welded,and then the plug-in welding.the plug-in soldering is done with wave soldering, it must be through wave furnace. The fixture is avoid surface mount components be assembled don’t fall when through wave furnace.

5.Panel drawing file

If PCB is made from panel,it also need provides panel file.Because the assembly manufacturers need panel drawing to make coordinates files.

How To Design Double Sided PCB


PCB (Printed Circuit Board) is an important electronic components, it’s base board of electronic components and carrier of electrical connections. it is called “printed” circuit board because it is made using printing technology.

Double-layer circuit board is layout wiring on both sides, but to use the two sides of the wire, must have appropriate circuit connection between the two sides. The “bridge” between such circuits is called via. The vias filled with or coated with metal holes in PCB, it can be connected with the two sides of the wire. Use PROTEL to draw double-sided pcb board, in TopLayer (top layer) to draw wire connection components, is in the top drawing board; select BottomLayer (bottom), in the bottom of the wire to connect components, is the bottom of the drawing board. Above is to draw double-layer pcb, meaning a pcb board in the top and bottom are painted wire. The double-sided PCB solves the difficulty of wiring in a single layer (which can pass through the hole to the other side), that is, both sides of the front and back wiring, the components can be welded on the front, can also be welded on the back, it is more suitable for use Single-side  complex on the circuit.

The two-layer board ground wire is designed as a grid-shaped frame, that is, the parallel lines are printed on the side of the printed circuit board, the other side is the vertical ground of the copy board, and then connected with the metal vias( resistance to be small).

In order to take into account each IC chip should be installed near the ground, often every 1 ~ 115cm place a ground line, so dense ground to make the signal loop area smaller, is conducive to reducing radiation. The ground network design method should be in the layout signal line before, otherwise it is more difficult to achieve.

Double-sided PCB board signal line wiring principle:
After Double-sided board rational distribution of components, followed by the first design of the network copy board power cord, and then layout important line — sensitive line, high frequency line, then layout the standard line — low line. The key lead line is best have a separate power supply, ground circuit, lead line must be very short, so sometimes design a ground line close to the signal line near the key lead line, let it form a smallest working loop.

Four-layer board top and bottom wiring principle is as same as double-sided PCB, also the first layout key crystal, crystal circuit, clock circuit, CPU and other signal lines, must follow principle the circulation area as small as possible.

PCB IC circuit working, above mentioned many times of circulation area, the actual its difference concept of differential mode radiation. Such as differential mode radiation definition: the circuit current flowing in the signal loop, the signal loop will produce electromagnetic radiation, because this current is differential mode, so the signal loop radiation is called differential mode radiation, the radiation Field calculation formula: E1=K1·f2·I·A/γ

In the formula: E1 — differential mold copy board circuit space γ radiation field intensity from the differential mode radiation formula can be seen, the radiation field strength and the operating frequency f2, circulation area A, operating current is proportional to, such as when the operating frequency f is determined, the size of the circulation area is the key factor that can be directly controlled in our design. At the same time, the working speed and current of the circulation are better meet reliability,not the bigger the better, the narrower the jump, the larger the wave component, the wider, the higher the electromagnetic radiation, the greater the power of its current must be large (the above has been pointed out), which we do not expect.

Several logic circuits are given below to meet the allowable loop area reference values for the radiation Class B standard. It can be seen that the faster the circuit switch, the smaller the area allowed.

The key to the line, if possible around it can be surrounded by the ground. After the PCB copy board is finished, the available ground will cover all the voids, but it must be noted that these covered ground lines should be shorted with the low impedance of the large strata, which can achieve good results (Note: Should meet the conditions, such as creepage distance, etc).

Some tips of double pcb board:

The use of automatic router to design pcb is attractive. In most cases, automatic routing of pure digital circuits (especially low-frequency signals and low-density circuits) will not be a problem. However, when attempting to use the automatic routing tool provided by the cabling software to do the wiring of analog, mixed-signal or high-speed circuits, there may be some problems and may cause extremely serious circuit performance problems. There are many things to consider about wiring, but the more troublesome problem is the grounding,If the ground path is started by the upper layer, the grounding of each device is connected to the ground via the cable on the layer. For each device in the lower layer, a ground circuit is formed by connecting the upper hole on the right side of the circuit board to the upper layer. The immediate red flag that the user sees when checking the routing mode indicates that there are multiple ground loops. In addition, the lower ground circuit is at a level. Can reduce the digital switching δi/δt the impact of analog circuits.It should be noted, however, that the two sheets have a ground plane in the lower layer of the circuit board. This is designed so that engineers can quickly see wiring when doing troubleshooting, often in the device manufacturer’s demonstration and evaluation board. But the more typical approach is to lay the ground plane on the top of the board to reduce electromagnetic interference.

    Steps of double side PCB:
    prepare the circuit schematic
    create a new pcb file and load the component package library
    planning circuit board into the network table and components
    automatic component layout
    layout adjustment
    network density analysis
    wiring rules set
    automatic routing
    manually adjust the wiring

PCB design experience of double side pcb board(embedded hardware experience):

    clearance spacing is better more than 10mil, high-density wiring is at least 5mil.
    The minimum line is 10mil out from welding line,no oblique line otherwise produce acute angle and unaesthetic.
    The main power line(current is relatively large)through the hole with two vias in parallel way to prevent a via failure lead to circuit board don’work.
    Power supply inlet capacitor with 100uf and 104 ceramic way the export capacitor capacity should be large enough to meet the circuit requirements (high current will not pull the voltage instantly). Turn off the diode from the power chip output pin closer the better.
    Power part of the resistance and capacitance to calculate the power, the package to meet the power requirements.
    A number of RF circuits, the  RF can be crossed on different layers, reducing interference.
    Pay attention to the lead position, to meet the schematic, not the same signal can be any position can lead.
    The same characteristics of the signal line wiring signal characteristics to the same, as long as the distance from the same length, the same number of vias.
    Will be some of the power supply of the decoupling capacitor filter capacitor can be placed on the back of the pin, save space and shorten the wiring distance.
    Wiring with latitude and longitude wiring, the upper and lower wiring is clear, but also to reduce the hole, reduce interference.
    When drawing the schematic, it is necessary to strictly calculate the rated current of the power chip rated current to meet the actual load requirements.
    When wiring, place the in – line elements around the core wiring area, which will cause interspersed and affect the latitude and longitude. To prevent interspersed, because the welding may scratch the line of the welding layer, so that when the welding pin may produce adhesion.
    Network chip under the ban copper
    Welding crystal crystal strict fall, because excessive shock will affect its performance.
    The four corners of the board are best rounded to prevent scratches.

LED Switching Power Supply PCB Circuit Board Design

Seven steps of LED switching power supply PCB circuit board design.

In the switching power supply design, if the PCB board design is incorrect will lead to too much electromagnetic interference.There are seven PCB design steps will great stable power supply.Through the various steps in the need to pay attention to analyze,step by step on the board to do a good job PCB board design.

1.From schematic to PCB design flow

Create component parameters—> Input principle netlist—>Design parameter setting—>Manual layout—> Manual wiring—>Verify design—>Review—>CAM output

2.Parameter Setting

Adjacent wire spacing must meet electrical safety requirements, and for ease of operation and production,the spacing should be as wide as possible.The minimum spacing at least able to withstand the voltage,the signal line spacing can be appropriately increase when the wiring density is low, the high and low disparity of the signal line should be as short as possible and increase the spacing. In generally, the distance should be more than 1mm between the edge of hole to printed circuit board edge avoid pad defect in the processing.when the traces are thin connected to the pads,the pad should design to droplet-like.the advantage is that the pads will not peeling and the traces are not be easily broken.

3.Component layout

Practice has proved although circuit schematic design is correct, if printed circuit board design is improper also impact the reliability of electronic equipment.For example,if two parallel lines of printed circuit board are close,will formed a delay signal waveform and reflection noise at the terminal of the transmission line.it will decline the performance of product due to power and ground considerations are not thoughtful.so should use the correct method in the design of printed circuit board.Each switching power supply has four current loops:

    ) Power switch AC circuit
    ) Output rectifier AC circuit
    ) Input signal source current loop
    ) Output load current loop

The input loop charge the input capacitor through an approximately DC current and filter capacitor acts primarily as a broadband energy storage.Similarly,the output filter capacitor is used to store the high frequency energy from the output rectifier and eliminating the DC energy from the output load loop.Therefore, the input and output filter capacitor terminals are very important,input and output current loop should be only from the filter capacitor terminals connected to the power supply.If the connection between the input/output circuit and the power switch/rectifier circuit can not be directly connected to the terminal of the capacitor,the AC energy will be filtered by the input or output filter and radiated into the environment.

Power switch AC circuit and rectifier AC circuit contains high trapezoidal current,these currents in high harmonic component,the frequency is much larger than the switching base frequency,the peak amplitude can be up to 5 times of continuous input/output DC current, the transition time is usually about 50ns.These two circuits are most susceptible to electromagnetic interference and must be routed before other wiring lines in the power supply.The three main components of each circuit filter capacitor,power switch or rectifier, inductor or transformer should be connected to each other adjacent to place, adjust the position of the components so that the current path between them as short as possible.

The best way to build a switching power supply layout is similar to its electrical design.The best design flow is as follows:

    Place the transformer
    Design the power switch current loop
    Design the output rectifier current loop
    Connect the control circuit to the AC power supply circuit
    Design input current source circuit and input filter design Output load circuit and output filter according to the functional unit of the circuit,all the components of the circuit layout,to meet the following principles:

 (1.)First consider the size of the PCB. PCB size is too large, the printed line is too long will increase impedance and decrease anti-noise ability,then increase the cost. If the printed circuit board is too small will lead to bad heat dissipation,and adjacent lines are vulnerable to interference. The best shape of the board is rectangular,with an aspect ratio of 3:2 or 4:3.

(2.) Consider the subsequent welding when place the components,the better not too density.

(3.)With the core components of each functional circuit as center,layout around it.Components should be uniform,neat and compactly arranged on the PCB,minimizing and shortening the lead and connection between the components,decoupling capacitors as close as possible to the device.

(4.) In the high frequency of the circuit, to consider the distribution of components between the parameters.In generally should arrange the components as parallel as possible.This is not only beautiful,but easy to assembly and mass production.

(5.)According to the circuit process to arrange the location of the various functional circuit unit,so that the layout flow, and make the signal as much as possible to maintain a consistent direction.

(6.) The first principle of the layout is ensure that the wiring rate,pay attention the fly line connection of mobile devices,put the connection devices together.

(7.)As far as possible to reduce the loop area to suppress the switching power supply radiation interference.

PCB Depth Control Routing Technology

PCB Depth control routing technology is the stepstair or groove shaped by partial routing in printed circuit boards(PCBs) manufaturing.It have great advantages in PCB assembly as the special shape of depth routing.

  • Use the step stair of PCB edge make card slot is favor for PCB fixed and installation.
  • It can avoid cold soldering as siphonic effect use NPTH stepstair slot.
  • Use the stepstair in middle of PCB apply in special module application,and degrade the height of PCB assembly,achieve refinement and miniaturization.
  • Use special size NPTH stepstair as resonant cavity of microwave signal,degrade signal loss and other special function.

There are two main methods of manufacture stepstair .

Depth control routing:

PCB depth routing

Filling and embedding cushion material.

PCB depth routing

We main introduce the control depth of stepstair and manufacture capability for depth control routing. And learn the control of adhesive,the pattern of stepstair rim and the hole’s reliability,size capability of stepstair.

The study for Capability

Mechanical milling:

For the stepstair PCB of mechanical milling,the precision can be +/-4mil is very high as equipment factor.As the stepstair slot is always in the depth of 0.2-0.4mm at some layers, it’s impossible to keep milling to particular layer, there have depth tolerance as the control principle and mechanical precision. At present,main use two ways to control the depth of milling in stepstair milling process.

  1. Optical rule control Z axis height:through Optical rule induce and control Z axis height to finish depth control routing.

PCB Optical rule control Z axis depth

  1. Through milling tool and copper layer form a closed loop path to measure and control the height of principal axis down to finish depth control routing process.The copper layer can be surface layer or particular layer. The precision of this method can be achieve to +/-15um,so the copper thickness of stepstair minimum is 30um,it risk copper remain at bottom layer after depth milling process.

pcb depth routing

 

Filling and embedding cushion material.

Filling cushion material to make depth routing for that stepstair is in middle of PCB center, main control the resin flow at side wall,slot bottom and notch,ensure resin flow stability at bottom, resin flow homogeneity at side wall and no resin flow at notch.

PCB depth routing-2

For this kind of stepstair milling PCB, all use FR4 materials and CORE+CORE to lamination. The prepreg thickness 20mil, the chip thickness 20mil. There are different size of stepstair,minimum 20x20mm,maximum 200x200mm,the depth milling is 1.0mm.

PCB Silkscreen Design Tips

Many PCB designers think the silkscreen legend is not as important as circuit,so they didn’t care about the legend dimension and place position.As a PCB expert, we should consider these particular details.There are some tips for those new PCB designers.

PCB silkscreen automatical machine

PCB silkscreen automatical machine

The standard design of PCB Silkscreen:

Oil printing: The minimum character width is 5mil, height is 28mil, the whole legend width is

16mil.

PCB silkscreen-1

Etching printing: The minimum character width is 8mil, height is 40mil,the whole legend width is 28mil.

PCB silkscreen-2

The Silkscreen don’t overlap with vias:

If the silkscreen printed on vias, vias will break the silkscreen lead to legend can’ be recognition.

PCB silkscreen-3

Silkscreen direction:

Silkscreen characters should follow direction order from left to right,from bottom to top. Do not use four directions when placed, this will lead to operator is very tired for degugging,maintenance, welding.

PCB silkscreen-4

Don’t place silkscreen on high speed signal track:

Dielectric related the impedance value, if place the silkscreen on high speed signal track will lead to dielectric become non-homogeneity then effect the impedance control.

PCB silkscreen-5

 

Heavy copper board silkscreen:

For heavy copper PCB,we should’t place the silkscreen place to the gap between copper and base material, as big difference of height of heavy copper PCB.

heavy copper PCB

PCB silkscreen printing should be neat, clear, no staggered except above mention.

Flexible Printed Circuit Board(FPC) Defect Detection Methods

FPC feature
Flexible printed circuit boards are more classification methods.According to FPC layer can be divided into: single-sided,double-sided,multilayer and Rigid-Flex PCB. Flexible printed circuit boards(FPCs) market share continues to rise as electronic products tend to miniaturization and lightness. But Flexible printed circuit boards(FPCs) frequently occur breaking, short circuit,line width incorrect defects in processing, feeding, placement and other production processes.This article mainly analyzes the flexible printed circuit boards defect detection method.

Flexible circuit boards(FPCs) have many advantages: good reliability,strong heat dissipation,easy installation and low comprehensive cost, it’s convenient for the high integration and high performance of electronic products.Different production process make it have many unique features.

) High density assembly,reducing the connection between different parts.
) Light weight,thin thickness can effectively reduce the weight of product,easy to carry.
) Can be folded and any bending.
The status of defect detection technology for Flexible printed circuit boards(FPCs)

The existing Flexible printed circuit boards(FPCs) defect detection algorithm is mostly derived from the Printed circuit boards(PCBs),but it limits by its own unique feature,Flexible printed circuit board(FPC) defect precision is higher,the test sample dimension is larger,the test sample is easy to deform,making printed circuit boards(PCB) defect detection algorithm can not directly apply in n flexible printed circuit board defect detection,need develop appropriate detection algorithm according the actual characteristics of flexible circuit boards(FPCs).

In order to solve the problem that traditional template matching algorithm is slow and accuracy is low, the flexible printed circuit board(FPCs) line defect be divided into two types: global defect and local defect,use eight-connected domain area method and the histogram matching method to capture the global defects of the image,on basis of this,use projector matching and correlation coefficient method to recognize the local defects.This method is faster and more accurate than the traditional detection algorithm,but relevant defect classification is not enough.

Analysis of defect detection method for flexible printed circuit board(FPCs)

Aiming the problem of template matching in the global scope,flexible printed circuit board(FPCs) defect detection is implemented considering the local range template matching method.Considering the contour-based template matching method,the template should have significant contour characteristics.Although the conventional lines on the flexible printed circuit boards are regular,but these is no significant shape feature,moreover,the conventional lines are distributed in the whole image, and the template matching time is too slow,it’s unfavorable for line detection.

Shaped lines are irregular shape on the flexible printed circuit board(FPCs),generally including LED lights,S-shaped round.Due to the shape of the peripheral,the layout is generally related to the style of Flexible printed circuit boards. For this type of lines,consider the template matching method to implement test: First through template matching method of steer the various contour lines on the whole Flexible printed circuit boards position,access to contour lines,and then based on morphological theory for defect detection.

The detection process of the special shape line is below:

1. Load the registration template related data information,including the registration template area,registration template contour information.

2. Using the area information carried by registration template,locate the template search space,and search the template instance in the search image based on the normalized mutual correlation coefficient metric principle.

3. Cut the contour area. The size of the profiled area is obtained by finding the smallest circumscribed rectangle of the shape template instance,since the shape template is a rectangular region,so the area obtained by the minimum circumscribed rectangle is the matching alien region. Any template matching can not make the two images perfectly aligned in space, there is a permissible range of matching. Flexible printed circuit boards(FPCs) local area of the deformation can be ignored,in the limited space on the shape of the template matching due to narrowing spatial range,shape template matching accuracy,the overall matching error of about 1/5 line width(3 pixels) range,is in allowable range.

4. Considering the accuracy error of shape matching,before the standard template image and the profiled area image are directly measured, the small-size structural elements are used to etch the alien-shaped area to remove the boundary difference effect.when the image is defective,the corrosion operation at this size can not completely corrode the defective line, so it does not affect the final judgment result.

5. The template image and the contour area of the regional difference set operation, access to the difference between the two areas.When there is no defect,the difference is 0.When there is a defect,the area difference set returns a binary image that is not zero.

6. The difference image is connected to the domain label,split the different defect block.

7. The area of each connected domain is calculated,and the center of the geometric dimension of the largest defective block is used as the center of the profiled area to output the defect inforation.

The FPC defect detection method is studied from the global and local scope,and the experimental results are drawn:

) Due to FPC imaging deformation,based on the global scope of the template matching line dislocation,can not correctly locate the image on the defect information.
 
) Based on the local scope of the template matching,first cut out the FPC board relative to the conventional line with significant shape characteristics of the line area,as a template source. In the search image corresponding to the template area near the instance detection,compression search space,and then based on morphological theory to detect line defect information.

In short, the circuit board as the most basic part of the electronic circuit, a great impact on the quality of the entire electronic products. Therefore, how to achieve accurate detection of circuit board defects is an important part of product quality assurance, and most domestic enterprises generally rely on artificial visual inspection FPC defect detection, high cost, low efficiency, and because there is no relatively standardized quality standards, the staff Subjective awareness or visual fatigue and other reasons prone to missed and false detection; some enterprises to introduce foreign excellent testing equipment, by the equipment prices, technical support, after-sales service and other limitations, it is difficult to adapt to modern technology, low-cost, high-volume production needs. Therefore, the development of a fast and efficient FPC defect automatic detection system to meet the domestic market demand is imminent.

What Is OSP PCB Surface Finishing

PCB surface finishing OSP is abbreviation of Organic Solderability Preservatives. It grows a layer of organic film on clean bare copper surface through chemical method.This layer film has features like anti-oxidation,heat shock,moisture resistance, it can protect the circuit board copper don’t be oxidized or vulcanized,etc.But the protective film must be cleared instantly in the subsequent high temperature welding,so that the exposed clean copper immediately combines into to a solid solder joints with melting solder in a very short time.
OSP PCB Surface
The process of OSP:

Degreasing—>Washing—>Micro-Etching—>Washing—>Pickling—>Pure water washing—>OSP—>Pure water washing—> Drying

OSP Principle:

An organic film is formed on the surface of printed circuit board,firmly protects the fresh copper surface,and also be anti-oxidation and pollution at high temperatures.OSP film thickness is generally controlled at 0.2-0.5 microns.

Features of OSP:

Good flat,no IMC between printed circuit board and OSP film.
Good wetting,allow welding of solder and circuit board copper direct welding.
Low cost,low temperature processing technology, it can reduce the use of energy when processing.
Extensive using, it can be used in low-tech circuit board,also be used in high-density chip package substrate.
The types of OSP

Rosin
Active Resin
Azole
Disadvantages

Appearance inspection is difficult,it doesn’t suit for multiple reflow(generally no more than three times)
OSP film is easy to scratch
Strict environment requirement for storage
Storage time is shorter
Storage method and time

Vacuum packaging for 6 months(temperature 15-35℃,humidity RH≤60%)

SMT site requirements

OSP Printed circuit boards should be stored in low temperature and low humidity(temperature 15-35℃,humidity RH≤60%),and avoid exposure to acid-rich environments.OSP packs are assembled within 48 hours after unpacking.
Suggest to use it in 48 hours and save it in a low temperature cabinet without vacuum packaging.
Suggest finish DIP within 24hours after two sides SMT(surface mount technology) completed.

Flexible Printed Circuit(FPC) Surface Finishing Technology

FPC feature
1.Flex Printed Circuit(FPC) Plating

It may have adhesive or ink pollution on exposed copper conductor surface after FPC coating process when FPC plating pre-processing. There will be oxidation,discoloration due to high temperature process.we must remove the contamination and oxide layer of conductor surface in order to obtain a good adhesion of close coating,so that the conductor surface clean.But some of these pollution and copper conductor is very strong combination,it can not be completely removed with a weak cleaning agent,so we need use a certain strength of alkaline abrasive and throw brush to dispose.Covering adhesives are mostly epoxy resin and alkali resistance is poor,this will result in a decrease in bond strength.Although not obvious,the plating solution may penetrate from the edge of the cover,severe peeling of the cover.the solder drill into the coating below phenomenon.It can be said that the pre-treatment cleaning process will have a significant impact on the basic characteristics of the flexible printed circuit board FPC.Must pay full attention to the processing conditions.

When FPC thickness plating,the deposition rate of electroplated metal is directly related to the strength of electric field,the strength of the electric field follows the shape of the line pattern,electrode positional to change, In general the wire width is more thinner,the terminal of the terminals is more sharper,the closer distance to the electrode is,the electric field strength is more strong,the plating thickness is bigger.In use of flexible printed boards,there are different widths of the wires in the same FPC lead to the coating thickness is non-homogeneity. In order to prevent it happening,can attach a shunt cathode pattern around the line,absorbs uneven current distributed on the plating pattern. Great guarantee the plating homogeneity of whole areas. So it must work hard in the structure of the electrode. Here is a compromise, strict control for the high requirement of plating, and relax control for the standard requirement.

FPC Plating stains, dirt just plated with a good plating state,Especially the appearance is no problem, But soon some surface stains, dirt,discoloration and other phenomena.Especially did not find any defect when factory inspection,found the appearance problem when user received and make inspection.This is due to insufficient rafting,there is residual plating on the surface of the coating,after a period of time slowly caused by chemical reactions.Especially the flexible printed circuit board, due to the soft and not very flat, it easy have a variety of solutions accumulate in concave areas. And then occur reaction and discoloration in that areas. It need fully drift in order to prevent this situation happening,but also to be fully dried.It can use high temperature thermal aging test to confirm whether it’s drifting sufficient.

2.Flex Printed Circuit(FPC) Electroless

When the line conductor need be electroplated is isolated and can not be used as an electrode,it can only be electroless plating.General chemical plating used in the bath have a strong chemical effect,chemical gold plating process is a typical example.Electroless gold plating is a very high alkaline aqueous solution.use this plating process, it’s easy occur plating solution flow into under coating layer,especially if the quality of the coating process didn’t be controlled well,bonding strength is low,it’s more prone to happen this problem.

The electroless plating of the replacement reaction is due to the characteristics of plating solution.It’s easier to drill into under coating layer. Electroplating with this process is difficult to get the ideal plating conditions

3.Flex printed circuit(FPC) Hot Air Leveling

Hot air leveling was originally developed for rigid printed circuit board PCB coating and developed technology,it also applied in flexible printed circuit board FPC surface finishing as this technology is simple.Hot air leveling is direct immerse whole panel into the molten lead tin bath and blow the excess soldering.This condition is very harsh for flexible PCB.If the flexible printed circuit board does not take any protection can not immerse in the soldering.Prior cleaning and coating flux flexible printed circuit board,fold the flexible printed circuit board into titanium steel wire mesh and then immerse in molten soldering.Due to the hot air leveling process have same risk that solder drill under coating layer.Especially when the adhesive strength is low between cover layer and copper foil, it’s easier to happen this phenomenon frequently.As the polyimide film is easy absorb moisture.the moisture absorption water will evaporate as rapid heat lead to cover layer bubble and peeling.So it must be dry and moisture-proof management before FPC hot air leveling.

What is the min trace width and space of heavy copper PCB?

New PCB designer always confuse how to build the min trace width and space for heavy copper PCB. Now PCB manufacturer engineer tell you how to do?

The min. trace width/space at inner layer :


The min. trace width/space at outer layer:

How to calculate impedance value before PCB fabrication

It’s very important how to calculate impedance value before PCB fabrication.

1.What is impedance?

Impedance is the total resistance of alternating current produced at known frequency.it’s resistance sum of signal layer and reference plane.

2. Impedance Type

2.1Characteristic impedance

Characteristic impedance is the ratio of voltage and current of a single wave propagating along the line.Signal produces reflection in the impedance discontinuity if characteristic impedance of transmission path changes when signal is in the transmission process.


2.2 Differential impedance

Trace carries the opposite polarity of two same signal waveform transmission by individual,subtraction the two differential signal at receiver. Differential impedance is the impedance between the two lines.


2.3 Even mode impedance

Even mode impedance is the impedance of one line when the pair is driven commonly.

3. The factors affect impedance


W—Trace width/space. The impedance decrease when trace width increase, The impedance increases when the trace space.

H1—Insulation thickness. The impedance increase when insulation thickness increase

T—Copper thickness.The impedance decrease when copper thickness increase

C1—Solder mask thickness.The impedance decrease when solder mask thickness increase

Er—Dielectric constant. The impedance decrease when dielectric constant increase

Undercut—W1-W2. The impedance increase when undercut increase.

4. Impedance calculate equation

Zo=[87/( Er +1.41)1/2]*ln[5.98H/(0.8W+T)]

For example: Calculate impedance 50 ohm for 4 layer board :

L1(TOPLAYER)                             1.9mil

                                 1080+2116 7.3mil

       L2(GND)                                 1.2mil

                                 Core 44.5mil

       L3(VCC)                                 1.2mil

                                 1080+2116   7.3mil

L4(BOTTOMLAYER)                      1.9mil

Board thickness:65*0.0254=1.6mm

Er=4.2       H=7.3mil   T=1.9mil

Ln =2.71828

Input equation:50=[87/( 4.2 +1.41)1/2]*ln[5.98×7.3/(0.8W+1.9)]

Result:W=11.6

How to choose PCB material?


1. Some comments for select PCB materials:

a. For electronic products, using FR4 epoxy fiberglass materials.
b. For high temperature application or flexible circuit board, using polyimide materials.
c. For high-frequency circuit, using Rogers or Teflon materials.
d. For high heat dissipation products, using metal core materials,like Al based material.

2. Five factors we should consider when select PCB material:

a. High transition temperature(Tg) degree materials is necessary for high speed digit PCB.
b. Low coefficient of thermal expansion(CTE) is necessary.Since the X.Y and coefficient of thermal expansion in the thickness direction is inconsistent,it’s easy lead to PCB deformation,and cause PTH hole break and components damage.
c. Requires a high heat resistance.General requirements for PCB to have 250℃/50s heat resistance.
d. Requires material flattening. SMT soldering require PCB warpage <0.0075mm/mm.
e. High-frequency circuits require high dielectric constant,low dielectric loss materials. Insulation resistance, breakdown voltage, arc resistance should meet product requirements.

3.FR4 property according to IPC-4101

The most popular PCB surface treatment

PCB surface treatment ensure circuit board had good solderability or electrical property. Due to the nature of the copper oxide form tend to be in the air, it’s impossible to maintain the original copper,so copper requiring additional processing. Below is the most popular PCB surface treatment.

HASL and HASL-LF

HASL is hot air solder leveling. It coated with molten tin(Pb) solder and heating the entire compressed air(blowing) level technology to form a layer of anti-oxidation of copper,but also provide good solderability coating.The HASL usually sinking in molten solder,air knife blow the solder liquid level before solidification.air knife can make solder copper to crescent-shaped with minimized and to prevent solder bridging.

OSP

OSP(Organic Solderability Preservatives) is a process of PCB surface-treated copper foil ROHS compliance requirements.It is grown an organic film layer with chemical methods in the clean bare copper surface.This membrane has oxidation resistance,thermal resistance and moisture resistance to protect the copper surface does not rust in the normal environment.It be required rapid clearance to flux in the subsequent high temperature solder.So the clean exposed copper surface can be combined with the molten solder joints immediately became solid in a very short period of time.

IMMERSION GOLD

Immersion gold surface is wrapped with heavy nickel electrical gold alloy,which can be long-term protection PCB. In addition it has great patience for the environment that other surface treatment processes do not have . Immersion Gold can also prevent dissolution of copper,which will benefit the lead-free assembly.

IMMERSION TIN

Since all current solders are tin-based, the tin layer can be matched with any type of solder materials. Immersion Tin process can formed planar copper-tin intermetallic compound, this feature makes immersion tin having a good solderability like HASL but without flatness problem. Immersion Tin board can not be stored for too long,Assembly must be followed the time of Immersion Tin.

IMMERSION SILVER

Immersion Silver is between organic coating and Immersion Gold, the process is relatively simple and fast.the silver is till able to maintain good solderability even exposed to heat, humidity and pollution environment.Immersion Silver does not have good physical strength possessed like Immersion gold because there no nickel under silver layer.

ENEPIG

ENEPIG(Electroless nickel electroless palladium Immersion Gold) is more palladium layer compared with Immersion Gold. Palladium appears to prevent corrosion caused by substitution reaction. It well prepared for Immersion Gold.Gold is closely covering the palladium,provide a good connection surface.

Hard gold plating

In order to improve the wear resistance of products,increase the number of plug of gold finger, hard gold plating surface is necessary.