Intrusive Reflow

07 January 2006

One method of soldering all surface mount and through components in a single operation is Pin In Hole Reflow, or Intrusive Reflow soldering, which has steadily gained importance over the last ten years. Basically all through hole locations to be soldered have solder paste printed over the holes prior to mounting the components. The volume required to fill the hole and provide solder fillets are defined by the stencil thickness, aperture size, often larger than the resist aperture size on the PCB. The speed of printing and the pressure can be adjusted to increase or decrease paste hole fill and it's a simple solution for soldering many through hole parts.

Assembly is generally done manually as a final assembly stage after all other surface mount parts have been placed. It may be done first to avoid any unnecessary movement of the surface mount parts. Odd form and surface mount placement systems can be used to avoid manual assembly. When complete, the assembly is passed through a reflow oven. Care needs to be taken when adjusting the profile as the connectors will normally be the larges mass on the design.

The major factors affecting the design engineer are compatibility of components with reflow temperatures and solder volume. Normal guidelines for pin hole assembly and soldering are the pin size plus 0.010" provides the finished hole size after drilling and plating and is the minimum hole size for round pins. The pad size is the maximum hole diameter plus twice the minimum annular ring plus any fabricators allowance. Care should be taken in discussing the capability of the hole size tolerance from the vendor. The final hole size after plating and any finishing should be specified on the PCB drawing or specification.

Reliability of the joints is equal to hand soldering and wave soldering. Trials conducted with tin/lead and lead-free in association with NPL have demonstrated the long term reliability of intrusive reflow.


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