What Is OSP PCB Surface FinishingPCB 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.
The process of OSP:
Degreasing—>Washing—>Micro-Etching—>Washing—>Pickling—>Pure water washing—>OSP—>Pure water washing—> Drying
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
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
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 fabricationIt’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
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 :
Er=4.2 H=7.3mil T=1.9mil
Input equation：50=[87/( 4.2 +1.41)1/2]*ln[5.98×7.3/(0.8W+1.9)]
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 treatmentPCB 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(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 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.
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 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(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.