In article , elson@pico-
systems.com says...

Fred McKenzie wrote:

In article ,
Ralph Mowery wrote:


I have not bought any yet and have been practicing witht he SMD on old
computer boards. That stuff seems a pain to work with compaired to the
'regular' tin/lead that I have been using for the last 50 years.

Ralph-

I sympathize!

The problem with using tin-lead solder with SMD, is that lead
"amalgamates" with the silver contacts of surface mount devices,
resulting in a non-conductive layer between the device and the circuit.

While tin-lead-copper solder may help, it would be better to use
lead-free. It may work better if you use a temperature-controlled iron,
capable of higher temperatures than your old iron.
Actually, if it amalgamates, you are fine. That essentially means it has
made an alloy. Pbf component leads usually do NOT have silver on them, just
Tin. Now, GOLD is a problem, a certain percent of gold dissolved into a
solder joint can cause brittle structures that fracture under thermal or
mechanical stress. They call this intermetallics. But, I've never heard of
this with silver. I've done tons of boards with SMDs using PbSn solder, and
had no trouble with it. One time ONLY, I got talked into trying gold flash
plating on the circuit boards, and had HUGE problems with joints that never
flowed, or became brittle. The fix was, desolder, lift the lead, scrape the
pad down to bare copper, tin the pad, fold the lead back down and solder.
UGGGH! Still gives me nightmares!

Jon

I have ran into the gold problem. Only "simple" cure I know of is to
just scrape it to the base material.
I never gave the solder any other thought before just use the 60/40 or
beter if around 63/37 and rosin flux, but now with that lead free stuff
comming in and components having to be made to the Rohs standards it is
time to do some asking about the mechanics of it. Seems there are about
half a dozen or more mixes now without the lead.

There is one other problem I have seen , not with solder, but tin plated
items. The tin "whiskers'. Probably will be a problem with the
tin/silver and no lead solder. I have read that the military and space
agencies still use the tin/lead to not have that problem.

I know I have to use a lot more heat on the irons on that lead free
stuff.