Ken,
The norm seems to be running the backside first, using either a glue
machine or screening on solder paste. More and more I see people getting
away from the glue printer and using the same process on both side of the
boards. During the second pass ( topside) they use fixturing to help protect
the bottom side parts. Some prefer a screen printer fixture that you can
switch fixtures on depending on the board being run. The fixtures are
inverse topography of the board so it surrounds the bottom of the board and
supports it. It can be run with or without vacuum. Others prefer a pallet
that runs all the way through their process. The pockets on the backside
also insulate the backside components and help keep them from reflowing
during the second pass. The fixtures feature hold downs that are flush with
the top side of the board so you can use it to support the board in the
screen printer. The pallets have to be insulative to avoid running the oven
to high to compensate for the extra thermal mass. If you would like I can
send you a sample. Give me a buzz:
Jim Gleason
972-494-1911
GSC
-----Original Message-----
From: Jorge Engenharia <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: Tuesday, April 13, 1999 11:56 AM
Subject: Re: [TN] Double sided reflow
>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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