I'm no soldering expert but is it possible that if the reflow profile was
borderline, the larger body might 'insulate' the unused paste area
underneath by several degrees causing it to remain in a semi-paste state
and over time, contaminate or migrate into other areas of the board? -Just
a wild thought... And shouldn't there be an IPC spec to cover this sort of
thing?
Ron Olinyk, CID
Mindlink Technologies
At 08:22 AM 8/9/02 +0100, you wrote:
>One potential problem is that the paste/solder underneath the component can
>push the body up, so that bad joints can occur.
>
>Another could be that the solder can be "squished" all over the place and
>potentially short.
>
>However, if its not been a problem with the assembly department and has not
>caused any returns then what the heck.
>
>The easiest way to prevent any problem would be to simple replace the solder
>paste screen with one that does not include the smaller
>footprints pads. And have the old one for when it is used.
>
>Matthew Lamkin.
>
> > -----Original Message-----
> > From: Steve Kingdon [SMTP:[log in to unmask]]
> > Sent: Thursday, August 08, 2002 9:28 PM
> > To: [log in to unmask]
> > Subject: Re: [DC] Land Pattern within Land Pattern (TQFP)
> >
> > Hi,
> >
> > Could someone enlighten me as to the potential problems of having reflowed
> > paste under the body of an IC. I am assuming if the paste contacted the
> > body of the IC, problems could occur...., but what sort of problems.
> >
> > We have one case where we have a SO8 inside a SO8W package. This was done
> > by engineers at my company, before they had PCB designers. I looked at it,
> > and didn't like it much. I asked our factory about it, but they have no
> > concerns, so I left it alone.
> >
> > Regards,
> >
> > Steve
> >
> >
> >
> > Date sent: Wed, 7 Aug 2002 13:54:42 +1000
> > Send reply to: "(Designers Council Forum)"
> > <[log in to unmask]>,
> > Dom Bragge <[log in to unmask]>
> > From: Dom Bragge <[log in to unmask]>
> > Organization: Klaxon IQA
> > Subject: Re: [DC] Land Pattern within Land Pattern (TQFP)
> > To: [log in to unmask]
> >
> > > > "Robert M. Wolfe" wrote:
> > > >
> > > > I have a situation where there is a need to provide a dual
> > > > footprint pattern for an alternate package.
> > >
> > > > Or would using an OSP coating solve the problem, but I guess that
> > > > coating once gone would leave exposed copper under that part.
> > >
> > > I would tread very carefully leaving just an OSP over bare Copper in the
> > > long term.
> > >
> > > When having done dual footprints before because of single sourced parts,
> > > I've found using some sort of HASL with two different paste masks to be
> > a
> > > good solution. Well, at least designing two paste masks & getting the
> > one
> > > first needed made - the other paste mask design is then your fallback
> > > solution. Also, if you are considering Alpha Levelling or other non
> > > tin/lead surface treatments, then look at the long term consequences. If
> > > you are doing it for a fallback solution, you could maybe do it by
> > > designing in the alternate footprint but leaving the second footprint
> > > covered by solder mask. Not quite as interchangeable, but you could
> > > generate two topside solder masks as well as paste masks & still be able
> > to
> > > use OSPs etc. It depends on how you'll handle the data for future use &
> > > whether you stockpile PCBs or get them JIT.
> > >
> > > I concur that you shouldn't screen solder paste *under* a component that
> > > doesn't use it.
> > >
> > > --
> > > Dom Bragge CID "The pink building"
> > > Production Manager Level 1, IBC Bldg
> > > KLAXON IQA Australian Technology Park
> > > bh+61-2-9209-4404 Eveleigh 1430 NSW
> > > fx+61-2-8374-5030 (Sydney, AUSTRALIA)
> > > www.klaxoniqa.com
> > >
> > >
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