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November 1997

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Subject:
From:
Eddie Brunker <[log in to unmask]>
Reply To:
TechNet Mail Forum.
Date:
Thu, 27 Nov 1997 10:13:42 GMT
Content-Type:
text/plain
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When designing double sided assemblies, it is a good idea to aim for laying
out a complex side and simple side as far as the Surface Mount is concerned.
The intention is to create a three thermal cycle process flow as follows....
                        SIMPLE SURFACE MOUNT SIDE
                        COMPLEX SURFACE MOUNT SIDE
                        WAVE SOLDER FOR PTH
The simple side shouldn't have large fine pitch QFPs, BGAs,Fine Pitch SM
connectors, Passives of large mass with few terminations which might fall
off during reflow. Hopefully the simple side would leave you with some green
spaces which are useful for supporting the assembly during the screen
printing of paste. It is vital that custom tooling is designed to give
maximum support to the Complex side for quality prints.
The temptation for complex sides is to build these first as they can be put
on a flat plate for maximum support in the printer, but consider the effect
of putting all your complex components through the Reflow upside down on the
second thermal cycle. The complex side build may require tooling to support
the assembly during the reflow operation. This depends on the nature of the
inner layers (copper planes with uneven distribution of copper) or perhaps a
multi circuit biscuit, both of which will cause warping in the reflow. The
tooling should be designed to hold the assembly flat while the PCB is at
it's glass temperature. Gravity should provide the neccessary down force.
Tooling should be designed to provide support without sinking heat from
critical areas of the top side during reflow.(This is an art in itself).
The PTH components should be layed to allow the design of a third piece of
tooling for Wave solder. The PTH components should have border areas around
the pins of about 3 - 4 mm which have no components (and no test pads if
possible) on the solder side. The tooling should mask the Surface Mount
components and leave apertures for the wave to reach the PTH leads. The
border area allows for a wall to come flush with the PCB and isolate the
Surface Mount.
Some people are unfortunate and have to struggle with a forth thermal cycle
and this involves the use of a Solder Fountain to assemble PTH from the
second side. Again a border area needs to be planned in to allow tooling to
fit around the components being soldered and protect the Surface Mount.

Thanks
[log in to unmask]

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