Thanks for your response, and sorry for the delayed return response. Unfortunately this product is in the prototype phase, and has not been built since last year, therefore, I don't actually have a process to evaluate. I would like to make any design recommendations before we go into pre-production (this is one of those long development projects). As far as the layout, there is nothing around the component to prevent adequate cooling. I am interested in what you are saying about reducing the crystal size. How would I check the crystal size, or even know what is sufficient crystal size for acceptable soldering?
Regards,
Howard Watson
Manufacturing Engineer
AMETEK/Dixson
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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