Typically, the volume of solder on a component lead is not sufficient in all
cases to provide a large enough solder fillet.  Was your board coated with
solder (hasl)?  This would make a difference.  there may have been enough
solder (hasl) on the board to provide for solder volume (thought not typically
well controlled for consistency pad to pad and across the pad).  Fine pitch
components are more sensitive to solder volume.  Too much will cause shorting.
A SnPb plated board with well controlled SnPb thickness and having all known
and controlled lead solder coatings may allow the  assembly supplier to obtain
good solder joints without additional solder volume in the form of "paste".

Most common approach is indeed to apply additional solder  ( a mix of small
balls/beads of solder suspended in flux) through a screen prior to component
placement.

In the end though, the real question is:

 Did your board meet the requirements of IPC-610?
 Do you have a component lead pull requirement and did you pass it?
 Did you have access opens at functional test?
 Were the products reliable in testing?

Changes in component suppliers may affect your assembly supplier's set-up (with
variations in component lead solder levels).

Jim Herard
KBL, Product Quality Engineering
IBM Microelectronics Endicott
t/l 857-7026

---------------------- Forwarded by Jim Herard/Endicott/IBM on 08-12-97 08:01 PM
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        08-12-97 06:18 PM
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Subject: [TECHNET] ASSEM: flux alone or flux with solder

Hi Technet

My company recently had a PWB assembled. It had about 75 SMT IC's some with
0.5mm lead pitch. The assembly house said they silk screened on flux only
and allowed what solder was on the PWB pads and the leads themselves to
effect the connection. Is this usual??

I thought that a slurry of flux and granulated solder particles was applied
to the board prior to placement and heating. They used a hot air heating
system and we had many cold solder joints especially in the PLCC sockets.
In about two weeks we're having more boards assembled. I welcome advise on
how/what to specify to the assembly house.

Thanks in advance -

Ray Klein    [log in to unmask]