Commentary – A P Products Meritec SMT Products Using Lead-Free Solder User Manual
Page 10
Meritec • 1359 West Jackson Street • Painesville • Ohio • 44077 • USA
10
Commentary
Other platings, board finishes and solder alloys were considered for inclusion in our tests.
The following is a brief discussion regarding these other materials:
• PCB Finish...Two popular finishes were considered for use in these tests, ENIG
(Electroless Nickel/Immersion Gold) and Immersion Silver. Each has unique benefits and
liabilities that warrant further discussion.
ENIG exhibits better surface wetting of the solder than Immersion Silver, but can suffer
from “black pad” if it’s not cleaned and handled properly during the PCB manufacturing
process. It is felt that as PCB manufacturers modernize their production lines this will
become less of an issue because cleaning equipment is continuously being improved.
Immersion Silver is not prone to “black pad” and is more easily implemented on older
PCB manufacturing lines, because cleaning requirements are not as stringent. This
provides some economic advantage. A liability of Immersion Silver is long term
formation of silver oxides. Though electrically harmless, they are nevertheless esthetically
undesirable.
Because we would be exposing samples mounted on these boards to extreme
environmental conditions in preparation for electrical testing of the “interface” end of the
contact, we needed to eliminate the possibility that no unknown variables would surface.
For this reason, we chose the Immersion Silver board finish over ENIG. It was thought
by many that, by the time of this writing, a lead-free HASL finish may have been
developed and accepted, but it has not.
• Contact Plating...Pure Sn plating of the socket contacts was considered and discarded.
Meritec has taken a conservative approach to pure tin on connector designs with pitches
less than 1mm. There persist, after decades of debate within the industry, uncertainties
regarding tin whisker growth. Our position is that a slight increase in the cost is worth
avoiding the risk of field failures in the future. For this reason, we chose pure Pd as a
contact finish.
• Solder Alloy...There are many solder alloys currently on the market, most of which are
patented. We chose to use the most popular public domain alloy because of it’s wide
availability.