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April 1999

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Subject:
From:
Jeff Hempton <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Tue, 13 Apr 1999 14:23:48 -0400
Content-Type:
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text/plain (146 lines)
     I have used board supports from Transition Automation very
     successfully in the past also. Their "BoardLok" system is a
     "vacuum-formed" rubber-like material which conforms to the entire
     backside, components and all, so you have 95%+ support from the
     backside during printing. I believe they are at
     www.transitionautomation.com

     Jeff Hempton
     United Technology Electronic Controls




Ken,
  Just to ad more info, the second side may need a special support on
screen printer in order to have acceptable board support, mainly if you
have fine pitch device on second side. This support is called "
dedicated workholder" by MPM guys, but you can built yourself using your
toolshop. Try MPM web site to see what a dedicated workholder looks
like.
  When we did double side reflow no problem were found using eutetic
solder paste.

Jorge Santana
Process Engr
Microtec - Brazil

> -----Original Message-----
> From: Stephen R. Gregory [SMTP:[log in to unmask]]
> Sent: 13 de Abril de 1999 12:20
> To:   [log in to unmask]
> Subject:      Re: [TN] Double sided reflow
>
> In a message dated 4/13/99 12:27:46 AM Pacific Daylight Time,
> [log in to unmask] writes:
>
> << Can someone help with the processing of a heavily populated SM PCB,
>  SM top and bottom which has to be reflowed both sides and has through
>  holes parts ( no wave solder ).
>
>  Any guidelines on how this should be processed would be of great
>  help ie what side is the high temp solder used on, most densly least
>  densly populated etc etc.
>
>  Regards,
>  ken Robertson
>  Telspec >>
>
> Hi Ken!
>
> I'm assuming that the reason you must do double sided reflow is
> because the
> board either has fine pitch or BGA's on both sides, am I correct? As
> far as
> using different melting point solders, I know there's some info out
> there
> that discusses that, but I really don't think that's a good thing to
> do. Just
> say for instance; what if somebody screws up and prints the wrong
> paste on
> the wrong side? You really don't need to do that anyway. Just about
> any
> component I can think of (as long as IPC guidlelines have been
> followed as
> far as pad layout and footprint) has enough surface tension to stay
> put when
> solder goes liquidous on the bottom...(provided your board isn't
> bowing and
> the reflow oven conveyer is smooth running).
>
> The exceptions to that are extremely heavy parts like ceramic or metal
> bodied
> QFP's and PLCC's, or parts with attached heatsinks. Sometimes Melfs
> will give
> you problems, or I've had problems with 4-leaded plastic body
> oscillators and
> certain filters too. Basically, suspect the parts that don't have much
> lead
> area to 'em. The figure I remember, and many people say this is pretty
> conservative, is that you'll have 30-grams of surface tension for
> every
> square inch of lead surface.
>
> As far as what side to do first, I usually leave the side that has the
> finest
> pitch on it for the last side. If ya got any other specific questions,
> let us
> know...
>
> -Steve Greory-
>
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