Designed properly, it’s actually okay if your machine makes contact with surface mount components.

One of the beautiful things about selective soldering machines (vs. wave soldering machines) is that they are easy to route around surface mount components mounted on the bottom side of the board. But it’s not always that easy. For example, what happens when the board designer ran out of room and placed surface mount components next to the through-hole components?

Believe it or not, contact can safely be made with surface mount components, so long as only one side of the component is reflowed at a time. This requires that the design of the board has the surface mount components perpendicular to the plated through-holes.

As shown in Figure 1, the path of the nozzle crosses directly over nine surface mount components. The area where the nozzle makes contact with the component will reflow the solder paste deposit, but the opposite side of the component will remain solid and hold the component in place. This can make one a little nervous the first time they try it, but rest assured, that component will stay put.

It would be a good idea to alert the board designers to this. Many designers like to use every square centimeter of the board, and knowing they can put surface mount components very close to plated through-holes, so long as they are perpendicular to the holes, will certainly make them happy.

Chris Denney is CTO at Worthington Assembly Inc; cdenney@worthingtonassembly.com.

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