Posted on 30 Jul, 2015 by Russell Poppe
The value of involving your electronics manufacturing services (EMS) partner in design for manufacture (DFM) is well established. Working closely with your EMS provider in this area can help to ensure a more cost effective product build, shorter lead times, and greater product quality and reliability.
However, like any meaningful partnership, the majority of these benefits only turn into reality through close collaboration. As the original design authority you will have a deep understanding of your client needs and market influences, which need to be communicated to your EMS provider. 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 is right for you, here’s a list of 10 common mistakes that are regularly made and how you can easily avoid them.
1. Leaving it too late
It’s recommended you involve your EMS partner as soon as possible, ideally when the very first draft of your PCB layout is available - and that you give yourself enough time for prototype and pilot builds. For all the checking you do on paper or with your PC, it is likely there will be a little "feature" that only shows itself when you’ve produced the first batch. These need to be flushed out before ordering all of the components for your first planned production build and committing your design for CE approvals and the like.
2. Incorrect PCB panels
The ideal panel will depend on the PCB and what processes the assembly will go through. If it’s too large or too small it might not fit on the production line. Particularly thin PCBs (e.g. 0.8mm) may need to be in smaller panels to avoid flexing. Lack of a waste strip may make 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. Missing fiducials can lead to alignment issues. Best practice advice is to allow your EMS partner to work with their PCB supplier in order to optimise the panel design.
3. Overcomplicating it
Unfortunately it does cost more if you have SMT on both sides of the assembly - quite often twice as much in fact - so, unless you really have to, don’t do it. This goes for through-hole components too. Spending a bit more time on the PCB design early on can help reduce the time, and therefore cost, during the assembly stages.
4. Wrong size or shape component footprints
It is well worth double checking that the component you have specified on your bill of materials (BoM) actually fits on the pads that have been designed on the PCB. Think too whether the body of the component will fit; often components are placed too close together, or too close to the edge of the PCB, which can result in them becoming damaged during break out or handling.
5. Vias in pads
It’s tempting when space is tight to do this, but to avoid having the solder disappear down the hole when trying to attach a component, the via needs to be "plugged" and maybe plated, which adds additional cost.
6. Too many component types
The number of component "lines" 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.
7. Mixing component sizes
As tiny components become more common there is a temptation to use them, but if they are placed next to larger components, which require more solder paste, then the process becomes a little trickier and may require more costly stepped stencils. This isn’t a "show stopper" but any complication is best avoided if possible, as it adds to the risk.
8. Inappropriate PCB finish
RoHS Compliant HASL is often specified as a standard finish, but it’s not the best for fine-pitch components. A silver finish has a shorter shelf life than most - perhaps ENIG would suffice instead? Consider if the finish originally listed is really the most appropriate and how changing to another could potentially save time and cost during the assembly stage.
9. Unnecessary components
When you get beyond the prototype build, do you really need those test points or that programming header fitting? Clearly, removing them will reduce ongoing costs.
It may be tempting to have components sharing a pad, or to leave out the solder resist between pads on fine pitch devices. Try to think where the solder’s going to flow - and the cost of failure and rework if unwanted short circuits appear.
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 it’s recommended you talk through DFM with your existing EMS provider to understand how they can support you.
Image by Lisa Brewster
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