Black pad results from dissolution of the surface of the Ni layer upon submersion in the acidic gold plating bath. Removal of too much nickel (especially along grain boundaries) leaves behind a phosphorus rich layer. When wetted with solder, the gold rapidly dissolves and the tin is left to react with the phosphorous wealthy layer. The ensuing poor bond energy may cause failure in the field when the solder joint is stressed. This determine additionally shows the preferential etching of the nickel along the grain boundaries, as noticed when the highest gold layer is stripped away.
It is an efficient end for ICT, the price is comparatively low, and it's a planar end so assembly of fine pitch parts is feasible. The risk of tin whiskers is often brought up as a concern, however usually the finish is simply too skinny to develop long whiskers. ImSn requires a somewhat complicated plating course of with a shower consisting of stannous halide and thiourea. Black pad remains a potential downside with ENIG when the gold plating process isn't properly controlled.
There is presently no floor end that satisfies the needs of all merchandise so sacrifices are often being made with whatever finish is selected. Selecting the optimum finish for a product application may be best carried out by determining what problems are most essential to solve and ensuring they are satisfied. This typically entails gathering input from multiple useful groups throughout the group to verify everybody’s point of view is taken into consideration. The object must be to pick a end that minimizes the overall cost (after contemplating the element price, the meeting value and the warrantee value). Wave soldering after meeting is listed as a primary difficulty with this end.
ImAg is a skinny movie of silver (usually 6-12 µ”) deposited instantly on copper. In 2003, earlier than RoHS took impact, the market share of ImAg as a surface end was below 2%. By 2008 this market share had shortly risen to close 17%.2 Silver has many favorable attributes however chief among these is the favorable solderability along with the power to simply probe on to the finish throughout ICT.
When the end is uncovered to elevated temperature, the thin tin layer can turn out to be almost completely converted to SnCu intermetallic (leaving little tin for soldering). Solderability can turn into an issue after the primary reflow cycle (or even after long term storage of the PCBs). High volume availability may also be a problem (until the PCBs are being produced in Europe). Many PCB outlets in the Americas or in Asia wouldn't have high quantity capability with this end. ImSn floor finish has seen a development rate similar to ImAg since 2003 (from 2% to 17%).