Rob,
We are also using intrusive reflow to simultaneously process SMT and Thru-hole components. Our arrangement goes as follows:
- Screen print solder paste to the single sided PCB using 0.008" laser cut stencil. - Apply adhesive dots to selected components. - Place SMT components (1206, 0805, MELF and MiniMELF, SOIC). - Invert the board. - Hand load through hole components. - Reflow.
The adhesive dots are required to prevent certain components, such as MELF diodes and the larger SOICs, from falling off the board as it passes through the oven. We have spent a lot of time experimenting with reflow profiles to perfect this. Another common problem early on was tombstoning, but by adjusting the volume of paste printed and the reflow profile we have been able to control this.
We have found the soldering of the thru-hole parts to be generally successful, although not 100% reliable; we still have to touch up joints post-process. However, much of this could be improved by reviewing our board design; our hole to lead ratios are not really ideal for intrusive reflow. In addition, certain thru-hole components (eg. electrolytic capacitors) can suffer damage in the heat of the reflow oven required to give good soldering.
We are still very much in the middle of a learning process ourselves at the moment. Nevertheless, intrusive reflow appears to be a worthwhile process for certain applications, and it's certainly worth putting in the time and effort to give it a try.
Best Regards Scott Davies
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