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NPI in Seven Steps – Design for Manufacturing and Test

This is the first part of our mini-series on New Product Introduction. If you missed the getting started post, read it here. In this post, we will talk about the first step in the NPI process: Design for Manufacturing and Design for Test (DFM and DFT).

No one likes broken products. Customers hate them. Engineers scoff at them. Manufacturers don’t get paid. It just sucks when a brand new thing just doesn’t work.

This is why, before even building the first one, the Contract Manufacture spends a good deal of time going over the design to make sure they can build it right and that they have a way to test all the functionality before it even leaves their doors.

Design for Manufacturing and Test

Design for Manufacturing and Design for Test Analysis

In this step of the New Product Introduction, the CM does a review of all the files you provided, from schematics to assembly documentation to labels. While PCBs are often a big part of this process due to their complexity, the plastic enclosures and custom metal attachments must also be examined carefully.

What the CM is looking for:

  • Can they physically build to your design – no matter how hard engineers try, physics still wins
  • Are components of the design within acceptable tolerances – placing PCB components too close can cause them to interfere or plastics don’t always come out at exactly 1.115mm thick
  • What can be automated and what needs hand assembly – human intervention is expensive and prone to error
  • How long do they think it will take to build – time literally is money
  • Once pieces are built, do they have a way to verify its right – they hate building bricks as much as customers hate receiving them

This process is broken down into two main sections:

  1. Design for Manufacturing where they will analyze the build
  2. Design for Test where they analyze how to verify success.

Design For Manufacturing

Design for Manufacturing, or DFM, is a review of the design and how easy it will be to assemble. This includes looking at how close PCB components are to each other, the shape of the footprints, and tolerance levels in the drawings. The primary goal of this step is to catch any problems with how the product would be manufactured before time and investment are made to attempt the build.

After all this careful analysis the CM will provide to your team a report of their requests. Usually, these requests fall into three buckets:

  1. Can’t Do – meaning they physically can’t manufacture this piece without changes
  2. Yield Loss – meaning that the tolerances are too tight or parts are too close and it may cause higher failures during build, and you must accept them or make the changes
  3. Cost Reduction – meaning that if you make this change it will reduce the part and assembly cost

Sometimes the feedback from the DFM is simple adjustments to reduce assembly complexity with no impact on the design such as:

  • Adjusting footprint sizes to prevent tombstoning or solder bridges
  • Increasing the distance between parts so the pick and place machine doesn’t bump other parts
  • Move all components to one side of board (if it only has a few) so they don’t have to re-run on the bottom side
  • Suggesting easier to place parts such as replace BGA with QFN
  • Adjust the board outline to fit more on a panel if that last one just barely don’t squeeze in

Pick and Place Machine

By Megger Ltd. ( [CC BY 3.0], via Wikimedia Commons

Others changes may require redesigning elements of the product that will cost too much to produce. These are the elements that CMs want to get early input on before either of you have invested too much time and money into the design. It may seem counter-intuitive that the CM is trying to reduce your cost, but they generally get the most profit when they finish your boards so the more boards you sell (by making them cheaper) the more money they will ultimately get.

Accept or Reject

Other than any changes that are labeled “Can’t Dos,” the CM’s list can be looked as suggestions. Typically very good suggestions, but it’s still up to your team if you want to accept each change or reject it and leave the design as is.

Sometimes you have to reject cost reduction changes because its the only way it will fit for the application, such as buttons and LEDs on both sides or size constraints mean parts have to be very close together. The point of the DFM is for the CM to make you aware of these decisions and sign off that you are consciously accepting or rejecting each one.

An important thing to remember here is that the CM is part of the team, their job is to manufacture your product as accurate and cost effective as possible, so listen to their suggestions as you would a teammate.

Design For Test

Design for Test, or DFT, is the second analysis that the CM will perform before building any product. This analysis will look at how well they will be able to verify the complete functionality and look of the finished good.

Just as your customer expects a working product from you when they receive it, the CM knows that you expect working product off their lines. So before boxing it up and shipping it to you, they want to verify that everything was built correctly and looks like new.

The DFT looks at how they will be able to verify that all the parts are placed correctly, interconnects are plugged in, and firmware was flashed. They will also look at how they can verify the assembly was done correctly and any manufactured parts (plastic or metal enclosures) were made to specification and free of blemishes.

To accomplish this they may request:

  • Test points on data lines and voltage rails so they can create a bed-of-nails
  • Test firmware and a way to communicate with it so they can verify functionality automatically
  • Requirements for power levels and current consumption during various parts of the test
  • Specifications for mechanical tolerances so they can measure with calipers
  • Documentation for normal operation so they know what they are looking for
Test pad on PCB for DFT

By Cschirp (Own work) [GFDL or CC-BY-SA-3.0], via Wikimedia Commons

Who Creates the Tests

At the DFT stage, the CM is looking for how they would test it, not how they would implement the test. This is an important distinction (one we will come back to in the Test stage of the NPI) as not all CMs have the capability to engineer the test code and hardware.

If they have design engineers on staff the CM may be able to build test fixtures (something to hold the assembly with pogo pins connecting to test points) and write test software (in something like LabView).

Other CMs will rely completely on your team to design everything including supplying test equipment and computers to run it.

Hopefully, the CM’s capabilities would have been known when they were selected, but it is during the DFT stage that everything should be clarified.

The Result

Once Design for Manufacturing and Design for Test have been completed, the manufacturer will provide to you a list or document describing the desired changes and the impact each element has on the cost and assembly complexity. Your team can then decide on what changes will be made (such as all the easy ones) and which are important to the final product (such as the look and feel) and are worth the extra cost.

Along the lines of cost and complexity, the next article looks will look at Part Availability and its impact on your price and schedule.

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