Posts Tagged ‘solder mask’

Misfits

Written by Rush PCB Inc on . Posted in PCB Assembly and component

If the shoe doesn’t fit, can you still wear it? You might have to if they are the only shoes available. In this case, the SMD packages for this PCB assembly application are actually wider than the PCB footprint itself. There are any number of reasons for this, from changes in component design to substitution issues, but we won’t get into that here. But the problem is that the leads actually overhang the SMT pads and extend onto the solder mask area (Figure 1).

 

These packages are too large for the corresponding footprints, with leads extending and overlapping the solder mask.

These packages are too large for the corresponding footprints, with leads extending and overlapping the solder mask.

 

This, of course, is unacceptable. But attempting to shorten or “snip” the leads won’t work either; the shear force could easily be too much for the package’s integrity.

The solution was to bend the pins in slightly so that they could fit onto the SMT pad without extending or overhanging off of the pads (Figure 2).

 

Misfit-PCB-Assembly-2

Bending the leads back slightly to fit within the confines of the pads is the only acceptable solution.

 

Certainly some stress and tension is applied in mechanically bending the leads, but not enough that we need to worry about it. And even though the lead is contacting the pad at a changed angle, there’s enough solder to create a robust solder joint. Remember that in the early days of SMT, some through-hole DIPs were snipped off and soldered to SMT pads creating butt joints, and these proved to be robust and reliable.

Misfit-PCB-Assembly-3

The bent leads solder to the pads just fine, forming robust solder joints, and meeting acceptability criteria.

 

An added advantage of not shortening the leads is that retaining lead length provides added spring-like flexibility for the lead to flex with thermal cycling, minimizing the possibility of solder joint failure due to thermally-induced stress. It isn’t much trouble, a good solder joint is created, and the part passes standard acceptability criteria because, in part, the leads are contained within the solderable pad area.

Read: Solder Mask Design Changes Help Prevent BGA Pad Lifting

By Roy