APPENDIX E: GUIDELINES FOR REFLOW SOLDERING
UNIFLOW
®
4 PULSED HEAT CONTROL
E-4 990-908
A. Heat is easily transferred away from the joint area to the large landmass, which is positioned
too near to the joint area.
B. Increased trace width and plated through-hole draw heat from the joint area.
C. The reduced width trace acts as a thermal dam and prevents any heat sinking of the pad.
D. 0.08” is the effective minimum area in which there must be no heat sinks if small trace heat
dams are used.
E. Equally sized small traces act as a thermal dam and ensure equal heating across joint area.
Traces leading from pads should be of equal width and be as narrow as possible. This design will act as
a thermal dam, and prevent excessive heat drain from the pad area during soldering.
Alternate designs:
For multi-layer boards, restrict the traces under the bonding area to the smallest width (signal) traces and
spread equally under the pads on the PCB. Any shielding on the PCB should have an equal effect along
the joint area.
Amount Of Solder Required On Printed Circuit Board Pad
The repeatability of solder deposit is critical in order to achieve good process control. In many cases a
certain amount of experimentation is required to achieve the ideal solder volume. A good starting point
is using a 0.006” screen print stencil, masked to give 40% pad coverage.
The amount of solder required on the pad of the printed circuit board is dependent on a number of
factors. The pad size and pitch determine the maximum and minimum solder quantity that can be
applied, using the screen stenciling process. Stenciled solder should be fused prior to the reflow process.
Small pad and pitch dimensions require less solder thus preventing bridged joints.
The flex design will also influence the volume of solder. Windowed flex and the exposed trace flex,
will stand a slightly greater solder volume in comparison to the single sided flex.
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