Sn/Ag Terminations with Sn/Pb Solder

This is a very timely topic, as we drift [buffeted by the combined wind of everyone down-stream in the supply chain ] on waves carrying all of us from the land of lead to the land of no-lead. I want to say there was a recent trade journal article published on the topic, but �

EASY PART

You surely can still use your standard Sn / Pb solder pastes and the current reflow and wave processes with Sn / Ag terminated parts.

NOT SO EASY PART

It might be worth talking time to discuss this change with your operations people. That way, they won�t be surprised if your inspectors decide that the silvered connections look different from the other connections on the board � * Low concentrations of silver seem to make connections shinier. * High concentrations of silver seem to make connections duller. * Well toasted connections with silver seem to get duller, probably from growth of Ag3Sn intermetallics that easily poke through the solder surface.

HARD PART - MECHANICALS

Making this material change obviously affects the properties of the alloys holding some of the components on your boards. Moving forward, you need to determine the impact of this change on the performance of your product. Let me pound a few stakes in the ground [that either may help baseline this or are so patently obvious that � ]. Anywho, I�d � * Ignore PTH and SMT components that are wave soldered, because of the mass of solder in the pot will displace the silver. * Continue to analyze impurities in the wave solder pot, placing redoubled vigilance on the silver concentration. * Focus on primary side SMT reflowed and all hand soldered components. * Get uncomfortable, if silver concentrations reach 3 percent by weight. [Although, as I say 3 percent, I recall someone babbling about a Sn94.5/Au4/Somethingorother paste the other day � whatever.] I�d still be nervous about silver GT 3.

HARD PART - ELECTROCHEMICAL MIGRATION

Over the years, silver has developed a bad rep because it migrates more than more commonly used materials [eg, copper, gold, nickel]. Not to say those other materials won�t migrate given the chance.

Electrochemical Migration (ECM). An unplanned electrolytic plating process. A film of polar solvent, often water, on a substrate surface provides a path for current flow between points with a difference in electrical potential. The process "grows" metal fingers between two traces, causing a short.

Methods for measuring / observing electrochemical migration are: * IPC-TM-650, 2.6.13 * EP&P, 10/99, p. 45

reply »