With a new CAD system that more easily/automatically differentiates between 
top and bottom side, we will be using longer pads for wavesoldering, which 
increases the probability of the wave contacting the pad, and reduces the 
process window for wave.  The pads are elongated to .060 vs .035 for reflow. 
 The distance between pads remains the same.
 ----------
From: TechNet-request
To: technet
Subject: SOT-23 lands for wave soldering
Date: Friday, March 07, 1997 10:17AM

Our Process Engineering group was recently asked to evaluate a pad
geometry for a standard SOT-23 transistor package.  The issue which
brought us into this was poor placement and soldering capabilities.

Our DFM guide states the distance from the top edge of the two bottom
pads to the bottom edge of the top pad to be 0.060" (0.040"w x 0.060"l pads. 

Center to center dimension of 0.100').  The Motorola recommended pad  spec.
 for the component states the same area between the pads should be 0.044"
 (.035"l x 0.031"w pad - center to center from top pad to bottom two
pads is 0.079").

The SOT is placed  bottom side and  processed with centered double glue
 dots, convection cured and wave soldered. Our DFM states the recommended 
pads
from Motorola are for screen printing solder paste and reflow applications.
I believe our DFM has the pads are too far apart regardless of the
process.

The question is for bottom side, glue, wave solder applications what
pad geometries have been successful for SOT-23's?

Side note: They have also stated that by elongating the pad from
0.060' to 0.080" for wave solder applications they get better wetting to the
leads? Ever heard of this being done?

Regards,
Jim
Process Quality Engineer

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