A relatively new procedure has been developed to enable a single reflow operation to solder both surface mount and conventional through-hole components on the same board. This is particularly desirable since in wave soldering of very fine lead pitch components, it is becoming increasingly difficult to achieve zero defects with respect to solder shorts despite the use of solder masks. This soldering process is referred to as Pin-in-hole re-flow (PIHR) or intrusive reflow assembly (Figure 8-16).
The-pin-in-hole PCB design incorporates both surface mount component pads and plated through-holes. Despite the use of through-hole components, the process utilises only reflow soldering. The solder paste printing process is performed in the normal way. Solder stencil thickness, squeegee speed and pressure are controlled to ensure that all surface mount component pads have the necessary volume of solder paste, and all plated through holes are at least 60 percent filled with solder paste.
After solder pasting, it is usual to place the surface mount components first and then insert the through-hole components. Through-hole devices are generally heavy and may be displaced during the automated surface mount component placement. Where through-hole components are hand soldered, care must be taken not to move the surface mount components or smudge the paste, causing solder balls to form on the PCB solder resist or mask (Figure 8-17). This may cause short-circuit connections between component terminations. Through-hole components selected for PIHR assembly should ideally have PCB standoff posts built into their body design, as shown in Figure 8-16.
This feature enables the remaining solder paste, solvents and flux to gas out from under the component and permits the through-hole vias to fill with solder. The pasted and populated PCB is then passed through the reflow oven, with care again taken to ensure that the temperature profile is in accordance with the solder paste and component manufacturers recommendations.
With respect to component reliability in the PIHR process, the leaded-component body will be subjected to a reflow temperature of approximately 230°C, or greater, whereas in the wave soldering process, the top of the board does not experience temperatures above 180°C because it is shaded from the direct heat of the solder wave by the PCB material. Therefore, leaded through-hole used in a PIHR process must be selected to ensure that the body of the component can sustain the higher air temperature experienced in the reflow process. Connectors, capacitors and relays may fall into this category.
