Dear TechNetters,
who shares problems with us concerning a formation of tin-iron
intermetallic needles during high temperature selective soldering? These needles
are forming electrical shorts between neighboring pins; for more details, see
the following paragraphs:
Driving the leadfree issue is automotive electronics due to
drastic increase in use environment temperature. We now have a case where
selective soldering of THT contacts is performed by chimneys which are loaded
with liquid solder by immersion in a solder bath, then raised to heat and fill
the joint. Using a lead free alloy (Sn-Ag-Cu), the bath temperature is set to
ca. 320C, as the heat capacity of the chimney is not overwhelming (but
simultaneous access is reducing cycle time considerably in comparison to a
miniature wave system). The bath itself is kept under a nitrogen blanket to
reduce drossing. The bath containment is made from stainless steel (Cr-Ni I
suppose), while the chimneys are made from tool steel, then pretinned. (By the
way, this method is used with tin-lead alloys just as well as with leadfree
solder alloys.)
It turns out that two different types of unwanted needles are
found after the soldering process. One type looks like cigar fish with its fin
tails and is found in the bath, on the side rails. The other type is found on
the solder resist, typically in between adjacent joints, forming an electrical
short. The first type was analysed to consist of mainly the solder alloy, the
second (and worse) type was analysed to consist of tin and iron. Further, the
bath showed an iron content of 6% after a couple of days of
operation.
Now there is a range of issues:
(1) formation of tin base
cigars (not the most critical at the moment)
(2) formation of tin-iron
needles (which is understandable if iron is freely available)
(3) enrichment of iron in the
solder bath - where is the acceptable limit?
(4) iron leaching from the
container or the chimney tool - how long until damage occurs?
(5) how much iron may a fresh
supply of leadfree solder alloy contain
(6) how can the adhesion of
the iron tin needles on the PCB surface be avoided?
Please give your comments, and if you encounter
similar problems, I would be very much interested in a direct exchange of
experience; you can access me directly at [log in to unmask].
Thank you very much
Thomas Ahrens, Fraunhofer
ISIT