Hi, Shen Zhiwei
Share with you on this.
¡°If the ratio of component weight in grams divided by the total
pad-mating area in square inches is less than 30, the components will not
fall off.¡±
Actually,Many different methods have been used to avoid two-step reflow.
For example, one method is to glue the bottom side components with
adhesive so that both sides can be reflow soldered simultaneously. This
method may save one reflow step, but it adds adhesive dispensing.
Dispensing adhesive after paste printing can introduce some of its own
problems.
Others have experimented with using solder pastes of different melting
points for the top- and bottomsides to avoid having bottomside components
fall off. Another method is to try to maintain different top- and
bottomside temperatures by adjusting top and bottom panel temperatures or
even blowing cold air on the bottomside while reflowing the topside. Just
imagine the stress and potential warpage in the board. I do not think one
has to make life so complicated. With some experience, it¡¯s easy to
figure out which components can go on the bottom and which ones should go
on the top. Zarrow and Belmonte1 derived a
weight-of-component-to-pad-mating-area ratio to determine if components
can be placed successfully on the bottomside. They established that if the
ratio of component weight in grams divided by the total pad-mating area in
equare inches is less than 30, the components will not fall off.
In a double-sided assembly, it is important to decide which side should
House the larger components(PLCCs above 84 pins or smaller PLLCCs attached
to heat sinks)that may fall off if the assembly is inverted. Obviously,
larger or heavier components(those attached to heat sinks)should be placed
on one side only, and this side should be reflow soldered last. There is
an added benefit to this option: The smaller components mounted on the
opposite side that are reflowed twice tend to self-align more during the
second reflow cycle. Self-alignment is generally not seen in larger
devices, with the exception of ball grid arrays (BGA). BGAs will
self-align even if they are misplaced by up to 50 percent. It should also
be noted that some very small components, such as ceramic crystals with
only four leads, may be considered ¡°large¡± because they will fall off
during a second reflow cycle if mounted on the bottomside.
Some double-sided assemblies containing only passive devices on the
secondary side have both wave and reflow soldering options for that side.
Wave, instead of reflow, soldering the secondary side is not and cheaper,
but the former may be desirable for many reasons. For example, via holes
can be fully filled only during wave soldering. The plugging of vias is
necessary to achieve the vacuum required for automated test equipment(ATE)
to work. If wave soldering is not used, the vias can be filled during
reflow by printing solder paste over them, Screens or stencils can be
ordered with this requirement in mind. Either option is acceptable, but a
choice must be made at the design stage.
Best regards.
FP Deng
Celestica Dongguan
Operations- Manufacturing Engineering Department.
Tel:0769-83394581
"Shen, Zhiwei (GE Healthcare)" <[log in to unmask]>
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2007-12-29 09:22
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Subject
[TGA] Need your help to find solution that big component drop when reflow
All
Do you have experience that the big component drop when reflow?
We have layout with big components on the bottom side, when we
reflow the top side, the big component would drop or find nonwetting
defect, would you like to share your experience on this?
Thanks and regards!
Jerry Shen
GE Healthcare Wuxi
Supplier Quality Engineering
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