| We've noticed that there are many different recommendations for pad sizes for various SMT parts. This seems to vary wildly from manufacturer to manufacturer for the same case sizes. Is there any standards that can used used to specify these sizes? Also, how does this vary based on reflow soldering versus wave soldering. | Paul: Let's get basic.
1 When you print paste to be reflow soldered, you contol the amount of paste available to make a solder connection by selecting the size of the aperture and thickness of stencil or screen and by printer process control. 2 When you solder SMT components with a wave, you have no control the volume of solder available to make a solder connection. It's virtually unlimited. You can control the volume applied to a connection by pad design and wave solder process conrol.
Why do you care about the volume of solder in a connection? Too solder reduces the reliability of solder connections. You know, that's why all the standards try to get you to produce concave fillets and limit the amount of flow up the sides of vertical faces of chip terminations, etc ... There has been studies to support this contention that you can obtain.
Should you care about the volume of solder in a connection? Dunno ...
1 A medical device firm I know allows no wave soldering of SMT devices. 2 A DEC (Compaq) employee presented on DFM to our SMTA meeting said they used the same pads on both sides.
Go get 'em.
Dave F
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