Choose Your Right PCB Surface Finish
Lead-free PCBs (Printed Circuit Boards) were first called for just for the compliance of ROHS (Restriction Of Hazardous Substances) by EU (Europe Union) that requires all the electronics have to conform to ROHS regulations. As a result, PCB fabrication starts to convert from lead to lead-free.
The lead in solder joints mainly derives from plated pins of components, PCB plated pad and solder. To ensure the lead content in solder joints conforms to ROHS regulations (mass fraction should be lower than 0.1%), surface finish applied on PCBs, therefore, has to be lead-free compatible. Right now, a couple of surface finishes have been generated for the achievement of lead-free fabrication and ENIG, Immersion Ag, ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold) and OSP are most widely applied.
Since each type of surface finish features its own advantages and disadvantages, it’s important to be clear which type of surface finish is optimal as far as your projects are concerned. Thus, this article will compare the four types of surface finishes primarily based on the analysis on application condition, cost, compatibility with lead-free requirement, shelf life, solderability etc. so that you’ll be able to figure out the optimal surface finish for your lead-free PCBs.
ENIG- Electroless Nickel/Immersion Gold
ENIG is short for Electroless Nickel/Immersion Gold and its structure is displayed below.
• General Description
As a lead-free surface finish, ENIG features some obvious ADVANTAGES, including long storage time, excellent solderability and flat surface. Its key DISADVANTAGE lies in relatively high cost and “black pad” risk.
• Black Pad
Black pad is actually a defect occurring to soldering joints with ENIG applied, which results from the fact that nickel layer suffers from severe erosion. Because the broken nickel layer looks grey and black, it is called black pad.
The leading disadvantage of black pad lies in the fact that it is difficult to be eliminated. Furthermore, it can’t be inspected by naked eyes. Therefore, it adds more threat to products’ reliability.
• Disadvantage Discussion
Non-wettability is the direct cause of black pad. Generally speaking, the ENIG-coating PCB suffering from black pad fails to withstand stress effect. As a result, once products go through high-low temperature cycle test, vibration test and daily plug in and out, solder joints will suffer from fraction, reducing products’ reliability.
b. Tendency to Errosion
Solder joints with ENIG applied tend to become eroded in humidity more easily because gold layer is so thin and features pinholes. Gold, however, doesn’t become eroded and it’s the nickel layer beneath gold layer suffers from erosion.
Im-Ag is a short form of immersion silver that aims to generate a silver layer due to the effect of solution. Instead of a pure silver layer, Im-Ag contains organic substance whose mass fraction is approximately 30%.
• Disadvantage Discussion
a. Micro Cavity
Micro cavities whose diameter is less than 0.05mm tend to be caused on silver surface. The cavities will dramatically decrease the strength of soldering joint, which is especially obvious when PCB suffers from shock. As a result, end products may become even failed.
b. Creeping Erosion
Creeping erosion is a leading defect as far as Im-Ag is applied as a surface finish. Because galvanic coupling is caused by the combination of exposed copper at the edge of solder mask and mass-area silver surface, electrochemical corrosion tends to be generated in humidity.
c. Silver Migration
Silver migration usually occurs to thick-film circuits or interior IC.
d. Color Changing
The color of surface of board coated with immersion silver surface tends to become yellow or black after being exposed by the air. Color changing takes place in the air mainly because small holes are available on the surface of silver and color changing takes place once haloid in the air reacts with the silver holes.
Besides such cause of color changing, sometimes surface color changes after soldering. There are two elements leading to after-soldering color changing: plating thickness and exposure time. It’s been testified that the increase of plating thickness is beneficial to color-changing resistance improvement and shortening of exposure time is also capable of relatively stopping surface color from changing.
OSP---Organic Solderability Preservatives
OSP is short for Organic Solderability Preservatives. OSP is actually a layer of organic film generated on clean copper surface in a chemical way. It is used to protect copper surface from being oxidized. Besides, it can resist thermal shock and wettability.
• Challenges OSP will Go Through after Soldering
Under the high temperature of reflow soldering oven, volatilization hardly takes place so that mass is lost by no less than 10%, which indicates that the smallest thickness can be used on OSP.
OSP won’t be decomposed under the temperature of 260℃. In the process, OSP is converted from solid directly to gas with no heat generated.
OSP tends to react with oxygen in the process of soldering.
As soon as OSP enters soldering oven, it’s easy for it to become rust color with its solderability deteriorated.
OSP tends to become hard to be eliminated with flux applied so that stronger flux should be used instead.
ENEPIG--Electroless Nickel Electroless Palladium Immersion Gold
ENEPIG works well with lead free and conventional eutectic solder alloys.This is the best way for gold wire bonding. Advantages for ENEPIG:
1“Black Nickel” free – no possibility of grain boundary corrosion of nickel surface by immersion gold
2 Palladium acts as an additional barrier layer to further reduce copper diffusion to surface, thus ensuring good solderability
3 Palladium completely dissolves into solder, without leaving an excessively high P% rich interface, exposing an oxide-free nickel surface allowing reliable formation of Ni/Sn intermetallic
4 Withstands multiple lead-free reflow soldering cycles
5 Demonstrates excellent gold wire bondability
- 6 Process costs substantially lower than electrolytic nickel gold or electroless nickel electroless gold.
- Many customers want to do ENEPIG,but they are not sure if the boards were used with ENEPIG? So,how to prove the PCB used ENEPIG?
One of our American college clients provided us what they saw in the PCB pads with a spectrometer.We made a lot of ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold) circuit boards for all of our customers in the world.You may would like to know more about ENEPIG,please click here.
Spectrum confirms the presence of Nickel (Ni), Gold (Au), and Palladium (Pd).
ENEPIG is capable of meeting more stringent requirements of multiple types of packages including THT (through-hole technology), SMT, BGA, wire bonding, press fit etc. What's better, ENEPIG is also suitable for PCBs with different packaging technologies. As a result, application fields ENEPIG can serve include aerospace, military ,Telecommunication equipments and high performance devices and medical industries with higher requirement of density and reliability.
Here is the PDF to prove ENEPIG was used in PCB,tested under Spectrum detector。