Design For Manufacture (DFM) Article

Working closely with your electronics manufacturing services (EMS) partner early in the design phase of your project will result in a more cost effective product, with shorter lead times, and greater product quality and reliability. As manufacturing experts, with experience across multiple market sectors and product types, your EMS provider is ideally placed to offer you design guidance when it comes to electronics manufacturing.
Whether you are already taking advantage of DFM with your current EMS provider or assessing if this additional service could benefit you, below is a list of 12 common mistakes that are regularly made and how you can easily avoid them.

1. No Consultation
Involve your EMS partner as soon as possible, ideally when the very first draft of your PCB layout is available. With consultation through your prototype and pilot build phases, you give yourself enough time to check the design and correct any part or process difficulties. This an important benefit before committing to ordering all of the components for your first planned production build and committing your design for approvals.

2. Incorrect PCB Panels
The ideal panel layout will depend on the PCB design and what processes the assembly will go through. Thin PCBs (e.g. 0.8mm) will need to be in smaller panels to avoid flexing during the population process. Lack of a waste strip may make PCB conveyor transport and handling during production and test difficult. Having breakouts in the wrong place may mean the assembly is not rigid enough, or difficult to break out without damaging components. Lack of suitable fiducials can lead to alignment issues and increased setup times. Best practice advice is to allow your EMS partner to work with their PCB supplier in order to optimise the panel design.

3. Inappropriate PCB Finish
RoHS Compliant HASL is often specified as a standard finish, but it’s not favourable for fine-pitch components and a silver finish has a shorter shelf life than most. Where possible consider ENIG RoHS compliant finish for assemblies with fine pitch devices or high complexity. Consider the choice of finish could potentially save time and cost during the assembly process.

4. Missing Solder Resist
It may be tempting to have components sharing a pad, or to leave out the solder resist between pads on fine pitch devices. These designs seldom provide a reliable soldering process which adds risk and cost of rework where misaligned components or shorts develop.

5. Vias in Pads
This practice has become more common place to accommodate PCB space constraints but more often than not, results in most of the solder paste on the pad wicking down the via creating a dry joint. To avoid this, via’s should be “plugged” and also plated where required, which unfortunately adds additional cost.

6. Wrong Size or Shape Component Footprints
Double check the component data sheets for the components specified on your bill of materials (BoM) to ensure the correct component pad size and shape has been considered. Also check whether the body of the component will fit at the required location, as often components are placed too close together, or too close to the edge of the PCB. Failure modes such as broken components; component misalignment; and solder defects can result from not adhering to best practise.

7. Stencil Design
Accurate paste print and volume is critical to ensure reliable and repeatable solder joints during the reflow process. You’re EMS and Stencil partners have much experience in this regard and can recommend specific aperture shapes and reductions to ensure just the right amount of paste is applied to avoid solder defects.

8. Complexity
Populating SMT and TH components on both sides of the assembly will add cost to the manufacturing of the product. Unless the design will be compromised, try to avoid double sided assembly as far as possible.

9. Mixing Component Sizes
As 0402 and 0201 sized components become more common in designs, they create manufacturing challenges where they are placed next to larger components, which require more solder paste. The soldering process becomes more challenging where different paste volumes and chemistries need to be considered to avoid process defects such as “Cold Joints” and “Graping”. This also increases the manufacturing cost as stepped stencils and more expensive solder paste types have to be used, which in turn adds risk to the repeatability of the process.

10. Excessive Component Types
The number of component “lines” in your BoM will affect the assembly time and price, so if you can re-use the same component value and/or package size then it’s recommended you do so. Passive components in particular have very little price variation so standardising package size, voltage, wattage and tolerance values can keep the SKU count down.

11. Unnecessary Components
Although having a test point available for every node during prototyping is essential for debug purposes, consider what will be required for the final product in terms of programming and testing and reduce accordingly. Removing any component that is not required for the function of the product will reduce ongoing product costs.

12. Integration
Most electronic designs are presented in some sort of enclosure and the final integration of your product can be equally challenging. Consider reducing the amount of mechanical assembly and fixtures required to integrate your product, this will save time and avoid additional tools. Also consider operator access points to key assembly areas, removing complexity with press-fit solutions will save cost without necessarily compromising reliability.

When it comes to electronic assemblies there are so many variables and therefore countless opportunities for things to go wrong. By working with your EMS provider at an early stage, many of the risks can be reduced. If you are currently working on a new electronics design, looking to upgrade a product already out in the field, or keen to understand how product quality could be improved, let Zyteq Technologies put their expertise to work in assisting you.