This layer can be so fragile that cracks can sometimes be found at this interface on Pb-free assemblies that had not even seen mechanical testing, as shown in Figure eight. The surface end chosen to be used on the printed circuit board is perhaps the most important materials choice made in the building of an digital meeting.
With the use of eutectic SnPb solder, the elastic modulus of SnPb was somewhat low and this allowed the solder to deform and take in stress on the joint. With Pb-free solder, corresponding to SnAgCu alloys , the elastic modulus is 25% larger. When stress is utilized, the solder does not deform as simply and the stress concentrates on the intermetallic interface. As a end result, when a Pb-free PCB with ENIG or ENEPIG end undergoes board strain due to deflection or a drop/shock occasion, solder joint failure happens at decrease pressure levels than with the opposite surface finishes.
However, after the soldering course of the copper often fails and this leads to a bad PCB. There are ways to deal with this type of copper failure.The first is to plate additional copper within the holes to allow for the etching process. The second is to plug the vias on the top and backside of the PCB with a photograph-imageable materials previous to the outer layers being etched. In this instance, the plated solder or plated tin is used as the etch resist when etching the outer layers. This plating is removed previous to applying soldermask, leaving exposed copper where solder connections are to be made.
At the end of the paper, an electrochemical migration corrosion failure mannequin of PCB was proposed. On circuit boards with finishes such as OSP, ImAg, HASL or ImSn, the tin based mostly solder bonds on to copper with formation of Sn-Cu intermetallics. In the case of ENIG and ENEPIG the tin types intermetallics with the nickel layer. In this case the intermetallic is usually Ni6Sn5 which is more dense and brittle than the copper primarily based IMCs.
These outer layer floor finishes are comprised of anticorrosive materials to protect the part mounting pads till soldering is complete. The particular finishes on this category embody HASL, natural coatings similar to ENTEC 106, ENIG, ENEPIG, immersion tin and immersion silver. ENEPIG is a type of floor finish accomplished on copper layer to guard it from oxidation and corrosion in printed circuit boards. This surface end improves the solderability of copper and can also be RoHS compliant. Results showed that, beneath the bias voltage of 12 V, the reverse migration of ions occurred.
For PCB-Cu, each copper dendrites and sulfate precipitates were found on the surface of FR-4 between two plates. Moreover, the Cu dendrite was produced between the two plates and migrated toward cathode. Compared to PCB-Cu, PCB-ENIG exhibited a better tendency of ECM failure and suffered from seriously short circuit failure underneath high relative humidity setting. SKP outcomes demonstrated that floor potentials of the anode plates have been greater than these of the cathode plates, and people potentials of the 2 plates exhibited a descending development as the RH increased.