Common Mistakes in Design For Assembly

In February, we looked at some of the common DFM mistakes that SAI helps its customers eliminate. Here are some of the common mistakes to avoid in design for assembly (DFA):

• Cable specification issues: IPC guidelines require specific wire gauges be used with specific terminal types. Use of the wrong wire gauge can result in wires coming loose or breaking off the terminal during normal use or maintenance activities. Lack of specifications such as failure to list tolerances or clarifying whether the cable length includes the connector or is simply the inside length are also issues.
• Assembly routing: The order in which cabling and tubing is assembled can impact both efficiency and product performance. An efficient routing sequences the cable harness assembly steps in a way that does not require the operator to have to work around previously installed harnesses. When tubing carrying air liquid is also involved, it is important to ensure the tubing is not accidently crimped by either its placement location in relation to harnesses or a harness tie down.
• Documentation formatting: the more complex a product, the more important electronic documentation becomes because it enables sorting and searching within the files. When documentation such as bills of materials (BOMs) are submitted as PDFs, they must be converted via optical character recognition (OCR) scanners. This can add errors to documentation. The preferred format would be an Excel file or a comma or tab-delimited text file.
• Missing BOM items: Sometimes consumables such as adhesives, zip ties or Lock-tite are not listed as a line item on the BOM. In addition to the quoting inaccuracy this can cause, it also can create confusion on the preferred product to use.
• Missing test and programming requirements: Sometimes functional test and programming requirements are listed only in the test instructions or not all steps are written down. There are often “tribal knowledge” issues that aren’t always conveyed to the contract manufacturer in testing.

Seven Cable Design Mistakes

As a company that specializes in being a one-stop shop of PCBAs, cable & harnesses and final assembly, Spectrum Assembly’s team works hard to ensure that projects are optimized for efficient assembly. Cables and harnesses are one area where SAI’s expertise comes in handy, since this part of the design is often not scrutinized as closely as PCBAs or the final assembly by customer design teams. Here are seven common mistakes the team at SAI sees:

• Wrong terminal or contact for specified wire gauge. If the contact is too large, the crimp will be loose and fall off. Conversely, if it is too small the crimp will be too tight and may damage the wire strand immediately or destroy it over time. In some cases, the terminal specification is correct, but an incorrect wire gauge or tolerance is specified.
• Male connector housing with female terminal. While this mistake is easily fixable, it can generate significant non value-added activity if not caught in documentation.
• Incompatible materials on header and cable. For example, specifying a gold-plated header on the PCB connector, but using tin on the cable terminal, can create resistance issues immediately and corrosion longer term.
• Cable documentation shows pinout but doesn’t identify connector. If the pinout only shows a single view and the connector isn’t identified, in the best case it slows down the new product introduction process and in the worst case it can result in an incorrect connector being used. If the pinout is incorrect it results in unnecessary rework.
• Incomplete or missing wire list. This is a frequent mistake with two-wire connections. It can cause quality issues.
• Proper crimp tool not specified. The tools used to crimp wire are specifically sized for the cable. Failure to specify the correct tool size or specification of an incorrect tool can create quality issues. IPC-A-620 includes a requirement for specification of crimp height and tool test.
• Insufficient electromagnetic interference (EMI) shielding or placement too close to sources of EMI. Insufficient EMI shielding or placement of sensitive cables near a power supply can create intermittent product failures.

SAI’s team is expert enough to be able to identify these issues as they occur. However, the best option is to avoid the issue in the first place by working with the intended contract manufacturer as early in the design process as possible. The focus that SAI’s team puts on ensuring cables and harnesses meet the requirements of IPC-A-620 is equal to the level of detail it focuses on in ensuring PCBAs are compliant with IPC-A-610 requirements.