Footprints for Success
The good designer acts as referee, heading off poor instructions and ill-advised engineering.
Design for assembly (DfA) is an ongoing struggle to balance assembly, fabrication and layout. The layout designer strives to create a product with ease of assembly in mind. To design the best assembly, the layout designer needs to understand the fabrication limitations of the components and the fabricator. The layout designer is a mediator between the requirements of the engineer, the fabricator’s abilities and the needs of assembly.
Footprint accuracy is the single most destructive mistake a layout designer or engineer can make. The component specs are not standardized. Footprint drawings are not always to scale. Some specs are drawn from a bottom view as opposed to a top view. Many components do not fit on the manufacturer-recommended footprint. The controlling dimensions are not always clear.
Using a CAD tool built to verify footprints can save schedules and rework. These tools have the ability to build a model to the dimensions of the component. The tool is able to overlay the model of the physical component over the footprint generated in the design tool. Using the latest spec ensures that the component is the latest revision.
The DfA check covers pin pitch (Figure 1), row pitch (Figure 2), pin type, component spacing, pin toe and heel (Figure 3), pin width (Figure 4) and overall assembly review. Any one of these items can delay a schedule or cause unattractive rework on a board. Many of the issues will scrap a complete lot of PCBs (Figure 5).
Pin pitch mistakes typically are made during the conversion from mils to mm or vice versa. The other error made is not identifying the correct controlling dimensions. The majority of mechanical drawings will include the controlling dimensions. Pin pitch is a cumulative error. On low pin count items, it rarely is a problem. The more pins a device contains, the larger the cumulative error becomes.
Row pitch does not suffer the same ability for cumulative errors, except on multi-row/column components. Ball grid arrays and connectors should be built in the original dimensions. Row pitch on quads can force a designer to use smaller pads on the corners or increase the toe while decreasing the heel of the solder joint.
Typically, pin type is a mistake that is made during the component lookup or a late BoM change. The manufacturer part specs are accurate between SMT and through-hole pins. The conflict arises on mounting holes. Many manufacturer specs do not indicate if the mounting pins are plated or non-plated. Press-fit pins require a tighter tolerance and should be noted in the fabrication and assembly drawings. Component spacing affects the initial placement and the amount of rework effort. BGAs require room for rework, or the surrounding components will need to be removed before the BGAs can be removed.
Pin toe and heel are critical for a solid solder joint. According to Texas Instruments’ solder pad recommendations for surface mount devices, the criteria for a well-designed solder joint are based on both empirical data and reliability testing. Solder joint strength is directly related to the total solder volume. A joint can be described by the solder fillets formed between the device pins and the PCB pads.
Pin width also is a factor to consider. The expected assembly process will help determine the width variations for the pin width increase. Boards that will be wave-soldered will need a wider pad than those that are reflowed. A major concern when making the pads wider is the solder mask webbing, which prevents shorts.
DfA is continuous process in which assembly, fabrication and layout must all work together. Understanding the requirements of each is key to creating a successful, smoothly running assembly. The layout designer plays an important role in this, acting as a mediator between the engineer, fabrication shop and assembly needs.
W. Scott Fillebrown is president and CEO of ACD (scott.fillebrown@acdusa.com). His column runs bimonthly.
