Design for manufacturability is a cornerstone in any Lean manufacturing strategy because it supports Lean philosophy in several ways.
First, when DfM recommendations are made and adopted as part of the new production introduction process, it contributes to faster production validation, since issues that might otherwise contribute to defects are eliminated before production qualification begins. Second, adherence to DfM guidelines optimized for a factory’s production process minimizes variation and the opportunities for defects driven by variation. Reduced variation may also contribute to reduced changeover time, which improves throughput. Finally, by minimizing the potential for defects, a robust DfM process reduces bottlenecks at inspection and test, and the concomitant non-value-added troubleshooting activities that can occur when higher defect levels are present, further improving throughput.
In SigmaTron International’s process, DfM recommendations are done during NPI, at customer request. Some of the most common DfM issues seen by the engineering team are addressed below.
Component land patterns. Land pattern choices are critical because they drive the spacing. The pad size must support the density requirement of product, as well as the formation of the correct solder joints. Our DfM review process uses a Valor part library (VPL) to verify the footprint of all the components specified in the bill of material (BoM) to match the land patterns used in the layout. Once potential issues are found, i.e., if the pad size appears incorrect for the specified component, the manufacturer’s datasheet is referenced. Most of the time, the problem is resolved by a slight change in the manufacturer part number (MPN) specified in BoM. In addition, the component-to-component spacing is also analyzed as part of the DfM review using the actual component’s body size and shape.
Solder mask design-related issues. Via-in-pad (VIP) can wick part of the limited solder out of the SMT terminations/solder joints. Similarly, a PTH via connected to the SMT pads that does not have the solder mask web in between has potential for insufficient solder joints and even open solder joint formation.
PTH vias once exposed by solder mask openings can be very useful as test access points for electrical testing, such as flying probe. Exposed PTH vias that are underneath and very close to low-standoff components, however, could get shorted during soldering operations.
Insufficient spacing of SMT and through-hole components. Selective point-to-point soldering is superior to manual soldering in terms of throughput and repeatability. The proximity of SMT components’ body and terminations located on the solder side of the through-hole components determines whether the manufacturing process should utilize either selective wave soldering pallets or selective point-to-point soldering equipment. The layout is also checked for sources of mechanical interference and to ensure that adequate spacing around the printed circuit board assembly is present for automated handling.
No thermal relief connection to copper plane. Through-hole components require thermal relief pads to connect them with the surrounding copper. This design element restricts heat flow, which improves soldering. When this element is not present and the component is connected directly to the copper pour, the high thermal conductivity of copper can cause the heat to dissipate from the pad, resulting in a cold solder joint and insufficient PTH barrel fill.
Tear-drop transitions to narrow traces. Narrow traces connected to PTH lands used for through-hole components could be thermo-mechanically stressed during soldering operation. When smaller traces must be used, tear-drop transitions or equivalent structures at pads can reduce stress.
Tooling considerations. With complex PCBAs and higher temperature processes, specifying the right tooling is also a critical component of DfM. We perform stencil design in-house because of the potential impact it can have on product quality, particularly in smaller PCBAs. One of the first steps is to change the land pattern shown in CAD to the actual component footprint. This involves replacing the CAD symbol with the footprint from the VPL. The Valor software pairs component footprint geometry along with the layout land pattern in order to optimize the aperture’s size and locations in the stencil. Some modifications from the software-generated design may be necessary. Typically the smallest component geometries determine thickness of the stencil, and necessary compensations to the aperture areas are needed for larger components.
A special carrier is often necessary to support the PCBA correctly through the assembly operations. Typically an SMT carrier is utilized to facilitate setup and to minimize warpage by supporting the PCBA during screen printing, pick-and-place, and reflow. Carriers also help ensure efficient processing in high-mix, low-volume production. They drive a repeatable, consistent process by minimizing the variability that can occur when the process becomes setup or operator-dependent.
Focusing on common DfM issues as early in the design process as possible cuts product development time and ensures the benefits of a Lean manufacturing process are fully achieved.
Yousef Heidari is vice president of engineering at SigmaTron International (sigmatronintl.com); firstname.lastname@example.org.