Highly efficient corrosion protection for plated pure tin surfaces

During hot and humid storage conditions plated tin surfaces tend to corrode, which will lead to tarnishing, increased contact resistance, reduced solderability and in certain applications to the growth of whiskers. In the present publication a chemical immersion treatment (called Protectostan), coated onto IC outer leads directly after the plating of tin, is described. This post-treatment is applied before the postbake (1 hour at 150°C) and will effectively prevent the corrosion of the tin surface. In order to mitigate the growth of whiskers on finished and assembled components during hot and humid conditions the Protectostan coating has to withstand the postbake annealing step and multiple reflow cycles. An extensive TOF SIMS (time of flight secondary ion mass spectroscopy) investigation has been undertaken to demonstrate the adsorption properties of the Protectostan surface layer before and after multiple reflow cycles and assembly on a PCB. Associated with the corrosion prevention the white tin layers will show strongly reduced to zero tarnishing even during the stringent pressure cooking conditions (16 hours). After 4000 hours at 55°C and 85% relative humidity the tin plated IC outer leads will still appear white and unchanged in colour. Components treated with Protectostan will retain a lower contact resistance as compared to non treated parts. This will lead to an improved behaviour and yield during electrical testing. Finally one of the greatest weaknesses of ICs with pure tin finishes, the deterioration of the solderability during hot and humid conditions, has been solved. A large experimental matrix and customer data will prove that the outer lead finish will still be solderable at lower temperatures and after steam ageing.