Howard I had a problem with the solder joint of a very large part's lead (by
comparison to other parts on the PCA) coming away from the pad with all the
solder attached to it because it was going through double reflow (the first
reflow looked and was great) and the part was not being cooled fast enough
to avoid terrific crystal growth within the solder. Are you seeing any of
these process problems? Is there something in the layout (p[roximity to
power/ground planes, large land pads or large parts) that is/are slowing the
heating/cooling rates? You might just have to remelt the joints with
soldering tips to reduce the crystal size and improve the structure.

Dorothy Lush

> ----------
> From:         Howard Watson[SMTP:[log in to unmask]]
> Reply To:     TechNet E-Mail Forum.
> Sent:         Thursday, August 30, 2001 6:58 AM
> To:   [log in to unmask]
> Subject:      Any MELF experts?
>
>
> Dear TechNetters,
>
> I have a situation involving a surface mount MELF Resistor, 270 ohm, 3W.
> During environmental testing, one solder joint separated at the pad
> causing intermittent failure.  Unfortunately, the technician repaired the
> solder joint so I have nothing to evaluate.  Based on information given to
> me, I suspect it was a mismatch of CTE between the MELF and the PCB
> (FR4,.062).
>
> Does anyone have any experience, pad design or CTE data,  with large,
> cylindrical MELF's?  This part is .5" x .156" dia.  and is custom
> manufactured by RCD.  We are stuck with this part in surface mount, as
> someone got a great "deal" on 100,000 or so parts, so it critical that I
> prove reliability (or lack of) with the solder joints.  In advance, I
> appreciate any information that can point me in the right direction.
>
> Howard Watson
> Manufacturing Engineer
> AMETEK/Dixson
>

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