Common DFM Mistakes to Avoid in Product Design

SAI’s TransferAssist team works with customers to ensure their projects are ready for outsourcing. The process includes a review of fabrication and Gerber files. IMG_8470Afterwards, the team makes recommendations for improving the design and/or documentation to better accommodate the requirements of a volume manufacturing process optimized for increased throughput with minimal handling.

In the design for manufacturability (DFM) realm, seven common mistakes stand out:

  • Panelization issues: this is an area where working with your contract manufacturer can be critical, as many contract manufacturers have specific guidelines on preferred printed circuit board (PCB) sizing and orientation for fast setups and maximum throughput. Additionally, the panelization strategy should be reviewed to determine if it is the most efficient layout in terms of minimizing wasted FR4.
  • Connector placement: Edge mounts must be kept on PCBs through processing. Depaneling tools cannot be used if a right angle connector is placed on a side of the PCB containing an edge mount. The only option for that type of layout is to depanel using manual force, which can negatively impact solder joint integrity. Consequently, it is important to consider panelization strategy in PCB layout when determining connector placement.
  • Documentation: Gerber files are often transferred as single-up designs instead of as a panelized design. This adds additional work during project transfer.
  • Stencil paste file errors: Optimum paste deposition often requires an aperture design that varies from the component footprint. Many Gerber stencil paste files are prepared 1:1, which doesn’t account for the impact of component mass in paste deposited via the screen aperture. Component manufacturers list recommended specifications and many contract manufacturers have an aperture design preference for specific types of components. For example, SAI has found that LED diodes are best placed when a triangular shaped solder paste aperture is used. Its team reviews the parts with critical paste deposition requirements prior to ordering the screen.
  • Fiducials: Fiducials are marks placed on the PCB to enable machine vision systems to understand the position of the component placement head relative to the PCB. Most PCB layout teams are good at adding fiducials to the PCB layout. However, some FR4 masking colors can make fiducials hard to read or invisible because the color of the mask is the same or lighter contrast than the fiducials. Machine algorithms can be adjusted to change fiducial brightness in most cases. Another option is to avoid masking over the fiducial. The most difficult color to compensate for is yellow. In this case, the “no mask” area should have a wide clearance for fiducials.
  • PCB finish mismatch: In some cases, a designer will specify a HASL finish for use with lead-free solder. HASL finishes do not have a flat coplanarity and in a lead-free reflow process can create coplanarity issues. An ENIG finish is generally preferred for use with lead-free solder. The team at SAI most often sees this issue when working with consigned materials.
  • Failure to utilize recommended pad layout size: Misalignment and other solder joint integrity issues can occur if the correct pad layout size for the specified component package is not used.

SAI’s Equipment and Processes Optimized to Support the Challenges of Flex Circuits

smtredoSpectrum Assembly, Inc. is experienced in placing components on wide range of substrate materials, including flex circuit assemblies. At SAI, medical and consumer products tend to be the primary users of flex circuit technology.

While SAI’s existing equipment easily accommodates flex circuit placement, strong focus must be placed on stabilizing the substrate during handling activities. Fixturing is an integral part of building high quality flex circuits. SAI utilizes 3D Solidworks, CAD or Gerber data to design carriers able to provide a rigid surface as the flex circuits go through stencil printing, pick and place and reflow. If an aqueous wash is required, a carrier must be designed for that as well. The goal is to utilize tooling holes in the fixtures to keep the substrate flat and rigid as it moves through the processes.

Understanding the end configuration, is critical to good flex circuit design. When a PCB layout designer lays out a rigid printed circuit board assembly (PCBA) it is fairly easy to understand the areas where design rules need to be followed in terms of clearances. With flex circuits, the designer needs to think three dimensionally in terms of locating points of interconnect, since the flex circuits are folded up and put in tight enclosures. If a point of interconnect is too close to a fold, it can create stress on the solder joint and result in a point of failure. Size of components may also create a design constraint. For example, a product with a long connector may be better designed as a rigid flex assembly than as a 100 percent flex assembly.

Equipment choices also matter. When rigid flex combinations or stiffeners are in use, there are two different heights which makes it impossible to screen print without specialized fixturing. Paste jet printers like the MY500 SAI uses are more adjustable and do not have that constraint because it allows us to tailor the amount of solder on individual components without the use of a stencil.

SAI’s Heller reflow ovens with KIC smart oven technology automatically verify in real-time that each assembly is processed in spec. This is critical with flex circuits because the carriers have a thermal mass impact and concomitantly increase the complexity of the reflow process.

The team at SAI also takes extra care in handling during secondary operations, since any flexing of the material puts stress on solder joints, particularly in the case of ceramic capacitors or other large ICs.


SAI’s TransferAssist Service Helps Companies New to Outsourcing Efficiently Start-Up Their Programs

engineers2When the team at Spectrum Assembly, Inc. tells a customer they are an extension of their manufacturing team, they mean it. One of the key ways this talk is walked, occurs in New Product Introduction (NPI) via SAI’s TransferAssist Service. Designed for customers new to outsourcing the service puts a Tiger Team in place to assess customer processes and address gaps in documentation and materials prior to the project transfer or product introduction.

The following case study illustrates the value of this service:

A company wishing to launch the next generation of a sports medicine product came to SAI. Their original product line had been mechanically simple and they wanted to increase functionality. This increased the product’s footprint significantly and required a much more complex mechanical assembly process. The customer wanted SAI to develop the manufacturing process instructions in addition to building the product.

Manufacturing process instructions are developed as part of the NPI process. The customer’s documentation for wire and cable assemblies, electronic assemblies, mechanical assemblies and cosmetics is transmitted to the Aegis system and used to create visual work instructions that combine the documentation assembly aids with notes made by SAI’s engineering team.

This project had several challenges. This was a box build job that included cables, printed circuit board assemblies (PCBAs), mechanical components, chassis, sheet metal, plastics and hardware plus fluidics, and air and water lines. Routing the tubing in a limited chassis space was a challenge. SAI’s team applied the experience it had in cable and harness assembly to address the tubing routing challenge, using same process model it used in developing efficient wire and harness routing to route the fluidic, air and water lines. Trial builds were done and the team developed a tubing cut list to ensure that all tubing was cut to appropriate lengths for the preferred routing. The customer watch the trial build process to ensure it met their requirements.

While computer modeling can be an efficient way for a design team to develop assembly drawings occasionally what looks great on the computer screen turns out to be an inefficient order of operations on the production line. Having a combination of computer modelling and hands-on trial builds ensures that nothing gets missed and that the order of operations is fully optimized.

SAI’s team also supported materials sourcing. When the customer’s engineering team originally created the bill of materials (BOM), they were simply looking for prototype quantities of the parts to support their proof of concept activities. Many of the custom parts were sourced through prototype shops. SAI’s team took the BOM and then identified suppliers with the best lead-time, best cost and a stable supply chain. In some cases, they recommended keeping suppliers that were tooled and competitive, however, they also identified alternate sources for suppliers that weren’t a good initial choice.

Finally, SAI’s team looked at the logistics side of the project. The product is used by sports teams and the crating needed to be both cost competitive, yet designed to support likely transport by end customers to other locations. Printing requirements, form, fit and function of all packaging was evaluated.

SAI typically provides some level of assistance to every customer. In this case, the team provided assistance over the customer’s entire product commercialization strategy, and truly lived up to our commitment to be an extension of their team.


Seven Cable Design Mistakes

IMG_8356As 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.