Oxidation of immersion silver coating on PCB

A. DEAR MIKE,
I HAVE CONDUCTED LOTS OF EXPERIMENTS FOR IMMERSION SILVER ON PCB SINCE YEAR 2001.
NORMALLY, THE IMMERSION COATED SILVER LAYER IS EASILY TARNISHED OR DISCOLORED BY HOT BAKING OR DIPPING IN sulfur-CONTAINING SOLUTION. THEREFORE, THE CHEMICAL PROVIDERS OF ImAg SUGGEST THE USER APPLY AN ADDITIONAL STEP AFTER IMMERSION SILVER COATING WHICH IS SO-CALLED SILVER ANTI-TARNISH AGENT. THIS CHEMICAL COULD PROTECT Ag LAYER TO AGAINST TARNISH AND DISCOLORATION IN THE AIR, AND MOST IMPORTANT, IT SHALL NOT HAVE WORSE EFFECT ON SOLDERING PERFORMANCE OF THE PCB.
TO STUDY THE PERFORMANCE OF IMMERSION SILVER COATING, YOU COULD DO IT BY THE FOLLOWING:
1. PLACE THE IMMERSION SILVER PCB SPECIMENS IN A DRY AND CLEAN OVEN @155 °C AT different TIMES, I.E., 1,2,3,4 HOURS AND CHECK THE APPEARANCE OF THE SILVER AFTER CERTAIN TIME BAKING;
2. IMMERSE THE ImAg PCB SPECIMENS IN A SOLUTION WHICH CONTAINS 5% WT Na2S WITH DIFFERENT TIMES, I.E., 1,2,3,4, AND 5 MINUTES AND CHECK THE APPEARANCE.
IN CONDITION 1, IF THE SPECIMEN COULD PASS 4H BAKING WITHOUT TARNISH (COLOR CHANGED TO YELLOW OR GRAY FROM WHITE) AND ITS SOLDERABILITY IS ACCEPTABLE, THIS COULD BE EQUIVALENT TO 12 MONTH SHELF-LIFE; SAME SHELF LIFE COULD BE CONCLUDED FOR 5 MINUTES IN CONDITION 2.
HOPE THE ABOVE COULD GIVE YOU ASSISTANCE.
BEST Regards,

A. Hi Jane,

Silver tarnish may affect solderability and aluminum wire bondability of immersion silver deposit. You may optimize storage condition (e.g. vacuum packing) or apply silver anti-tarnish after immersion silver to prolong shelf life.

Regards,
David