Vinit

I have seen this particular problem before.

From what you say regarding baking the PCB it appears to be partly due
gasses escaping from under the Surface Mount pads.

Try decreasing the ramp rate on your reflow oven to below 2degrees per
second. I know this is quite a shallow ramp but it can work. Alternatively
put a soak plateau in at 120 - 140 degrees for 30 - 60 seconds, (This 120
degree plateau used to be common when reflowing SMD's but with the increased
use of forced air convection ovens and new paste technologies we are told
that we don't need it but it does help). The reduced ramp rate will expand
the gasses at a slower rate so hopefully the solder wont be displaced, the
shallow ramp will also stop the solvent's in the paste from evaporating to
fast and blowing solder paste over the PCB.

Secondly you may be depositing to much paste on the pad. try reducing the
apatures in the screen. A joint will only hold a certain amount of solder,
depending on the surface tension of the molten solder (flux related), so the
excess solder will bleed away from the joint.

Thirdly, the placement pressure could be to large. This will cause the
solder paste to be pushed under the device. This paste that is under the
device will then reflow and escape from the side of the device.

P.S. I don't know why the solder ball always ends up in the middle of the
device. Maybe someone else can answer that.

===== Original Message from TechNet@PAT [log in to unmask] (TechNet E-Mail
Forum.) at 10/11/98 17:19
>I have recently been facing a problem after reflow soldering. Almost every
>chip component (approx. 60-70%) has a solder ball attached to the component
>on one side between the solder joints. This ball detatches if moved with a
>tweezer or something sharp.
>
>The paste quality has been checked, reflow profile has been fine tuned and
>checked, PCB has been baked to take care of any moisture entrapped. But the
>problem still remains. However, after baking of the PCB the occurence of the
>problem has reduced (to approx. 20-30% of chip components)
>
>If anyone of you has ever encountered this problem and knows of a possible
>cause and the solution to the same, please forward it to me.
>
>Regards
>Vinit Verma
>Bharti Telecom Ltd., Gurgaon, India
>Tel: +91-124-372721,22,23 (O)
>        +91-11-7871747 (R)
>Fax: +91-124-372724 (O)
>        +91-11-5783571 (R)
>
>
>--------------------------------------------------------------
>Bharti Telecom Limited, Gurgaon, India
>
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Mark Austin (Process Engineer)
ACW Technology Limited
Hylton Road
Petersfield
GU32 3XX
ENGLAND

TEL - +44 (0) 1730 300000
FAX - +44 (0) 1730 266045

Email - [log in to unmask]

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