Re-capping:

Bill Davis says that the 85/15 Sn-Pb lead finish is typical, my profile
probably needs to be raised a little.

Dave Hillman says that they sometimes specify a eutectic Sn-Pb finish, but
that he thinks the problem with my 85/15 parts is likely surface oxidation.

Steven Sauer is not sure of the typical leadframe tin-lead composition, but
says that eutectic makes good sense, although the device suppliers may be
trying to reduce that nasty old lead. (My guess is they care a lot more
about cost reduction than the environment OR solderability).

Oliver Kierse says that 85/15 Sn-Pb is indeed a typical leadframe finish spec.

Background:

This is a QFP208 part, and is soldered using a no-clean paste (Alpha
Ultraprint 78) in a convection reflow furnace with air (BTU VIP 70) on a
SMOBC HASL PWB (catch those acronyms?). Our peak is presently 215-220 °C.

The assemblies in question look good except for the part in question, which
has shallow fillets and marginal wetting.

My opinion is that the parts have inferior (to our normal component)
solderability, and that a ceramic plate test or other sensitive
solderability test would confirm this.

Although I find no specification violation, it would seem logical to me
that normal practice of having 20°C higer peak than the liquidus temp would
require about 230°C reflow, higher than typcial profiles we use here.

The issue of what the normal lead finish _should_ be is my quest here.

IMO, the solderability of our run-of-the-mill parts (mostly coming through
distribution) with our no-clean paste is good enough. Palladium parts are
typically pretty dicey. To be frank, I had suspected these parts had Pd
finish until the manufacturer told me 85/15 Sn-Pb.

Any further relevant facts would be appreciated.


cheers,


Jerry Cupples
Interphase Corporation
Dallas, TX USA
http://www.iphase.com

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