¡°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
[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