The two key factors to achieve highly reliable solder joints are PCB design and construction. As such, if double-sided mounting is used, boards with exposed pads must never be placed opposite one another. This can cause solder joints to fatigue earlier than it’s designed to be and will also stiffen the assembled PCB. This board stiffness can also significantly influence the reliability of the solder joints.
When it comes to the overall process of printed circuit board assembly, or PCBA, a lot of factors affect the board reliability and assembly yield. The major ones include the design and materials of the stencil used and the solder paste itself. Other factors are the component placement and reflow oven process. To further achieve the optimized printed circuit board assembly result, taking into account the customer’s SMT equipment, actual PCB and product-specific requirements might be necessary. It is also equally important to know if the temperature profile utilised will form the appropriate solder joint.
In addition, the thickness of a stencil should match the requirements of all the PCB components as well. When a certain stencil thickness does not meet the needs of other packages on the same board, then the use of another one has to be considered. And if you want to ensure that a uniform and adequate solder paste is transferred to your PCBs, consider using laser-cut or electroformed stencils. Also, a good paste release can also be achieved by rounding the corners of the apertures.
These apertures must have the same size to that of the metal pads on the board. Any stencil placed in these thermal pads have to be segmented into smaller openings. The best way to segment largely relies on the location of vias, if any, as well as solder resist layout on the pads. In this case, the stencil openings need to be arranged in places between the vias. While there are other PCBA processes involved, these factors are very important not to miss out in performing the printed circuit board assembly.