The Influence of PCB Design on Solder Defects in the Surface Mount Manufacturing Process

There are certain solder defects that can be greatly mitigated during the PCB and Panel Design Process. Understanding what not to do is important to make sure that the design of the PCB and Panel does not increase the chances of solder defects beyond the normal defect rate of a particular manufacturing process. Identifying sources of defects is easy once you apply certain basic principles.

1. In a perfect world, all the terminals of a given Surface Mount Device (SMD) will achieve liquidus temperature as near as possible to simultaneously during the reflow soldering process.
2. Bottom Termination Components (BTCs), especially those with large or irregular land areas need special treatment. Undesirable effects from failure to outgas the volatiles in the solder paste can create opens or voids that may create hot spots on the SMD.
3. The effects of via placement and solder mask cropping on an as-built PCB must be considered.
4. Circuit orientation in a panel can mitigate or magnify the influence of lead-lag thermal gradients.

The easiest way to understand the effect of liquidus temperature is in a two terminal SMD component such as an 0402 chip resistor. If a trace is being tied to a ground plane on one side of the SMD and there is a 5 Mil trace on the other side of the SMD, the side with the 5 Mil trace is going to achieve liquidus temperature first and the SMD will begin to move in that direction. This increases the possibility of a non-compliant component shift or even tombstoning. This effect is more prevalent the smaller the component size is relative to the solder volume on a particular pad.

Pad and pin escape geometry can regulate thermal gradients between terminals of an SMD. Bottlenecking pin escapes is an easy way to thermally balance a SMD, or conversely increasing the trace width to correspond to other trace widths on the device when possible. These techniques can be applied to many types of SMD to one degree or another and mitigate thermal gradients and increasing the chances of reducing the chances further with a well-designed reflow profile.

While there are other considerations when designing PCBs with Bottom Termination Components (BTCs), the two primary areas of focus should be on using Non-Solder Mask Defined (NSMD) pads and targeting a solder volume of 50%-70% on the ground pad versus the solder volume of using a 1:1 aperture. The volume and geometry of the apertures will also be influenced by vias resident within the ground pad.

It is typical that these features related to BTCs are left to the assembly house to implement on the stencil, but if the designer has correctly designed the PCB, the possibility of introducing variation to the product when assembly location changes is eliminated. IPC-7093A Design and Assembly Process Implementation for Bottom Termination SMT Components is a good resource. Caution should be taken when following recommended pad geometry from certain datasheets as they advise placing solder mask within the ground pad area which can lead to problems during reflow.

When determining the influence of layout on solder defects, it is good to remember that it is likely that where the solder mask is most needed is where it will be cropped. Particularly in the areas of the PCB that are likely to have the solder mask cropped in the as-built PCB, all pads should be viewed as Non-Solder Mask Defined (NSMD) pads. Cojoined pads and traces equivalent to or greater than the pad width will artificially increase the surface area of the pad will starve the solder joint and lead to a non-compliant solder joint or perhaps an open. Bottlenecks and other tactics including the use of the silkscreen as a solder dam can be employed in certain cases. Stencil design can also mitigate the deprivation of solder volume but may also increase the likelihood of a shift during reflow.

When laying up a multi-circuit panel, it is important to design the panels so that as much as is possible, each individual circuit will be in the same heating and cooling zone of the reflow oven. This is especially important with larger, long, and narrow PCBs that could be in as many as three zones at the same time if oriented incorrectly. In the case of a PCB that is three inches by sixteen inches, the panel should be laid up so that the three-inch ends are the conveyance rail ends; this will ensure that each individual circuit is as thermally equal as possible during reflow and cooling.

Every design is unique and often tradeoffs must be made to achieve an optimum PCB design. Whenever possible it is best to get input from the manufacturer of the materials, components, and the assembler of your product to make certain that risks to the manufacturability and long-term reliability have been mitigated. Freedom CAD offers DFM services along with PCB Layout to help make certain you avoid any potential problems with your design. Talk to an expert, contact us today.