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The Importance of Design for Test

Design For Test (DFT) is a process businesses use early in the product development lifecycle. The goal is to help evaluate the design of a given printed circuit board (PCB). Based on the board’s circuitry and connections, DFT’s emulates the product’s designed behavior. To accomplish this, software tools are used to ingest the board drawing data (CAD information). Once processed, reports are generated with details of the given board.

Hopefully, the result comes back with optimal test access for the PCB; however, that tends to not be the case and thus why DFT is such a valuable part of the product development process. Companies must decide whether a current PCB design is taking advantage of all potential test access. If not, they may elect to do a board re-spin because there is opportunity to improve the test access of the PCB.

Performing tests on PCBs are becoming increasingly difficult due to factors like:

-PCB’s becoming increasingly dense with components/connections
-Device speed increasing
-New SMT techniques
-Other changing PCB parameters and requirements

However, neglecting to put forth the best test strategy and test access can cause major repercussions for manufacturers and end customers later in the process. Therefore, ensuring a proper DFT early in the design phase is critical for new technology products.

Example of DFT report (using sample data)

Examples of DFT results include:
-Test Access (how many multi-pin and single pin nets can be tested)
-The size of test pads on the top or bottom side of the PCB
-Quantity of test points on the board
-If there needs to be any changes in circuit or schematic designs
-Proper clearance on board (between components, between test points, and soldermask)
-Listing board locations where test points can be added to improve coverage

Benefits of running DFT in preliminary designs vs. final design
-Be aware early on of potential design issues leading to structural integrity issues of PCB
-DFT reports providing insight into the PCB of where test access is currently not possible
-Enables design teams to re-evaluate the layout in order to improve upon test access for final PCB builds
-By catching potential design issues earlier in development, the cost is much less than when mass production of the PCBs start

Example Scenario of not having great test access
Company A is going through next gen product development and is manufacturing a PCB going into the final assembled product.

Company A PCBA costs: $150.00/unit
Company A manufacturing plan: 2,000 PCBs built/month
Company A PCB costs each month: $300,000/month

Company A fully assembled product cost: $900.00
Company A manufacturing plan of assembled products: 2,000
Company A assembled product costs: $1,800,000/month

If there is a 1% fallout of the assembled product, it costs Company A:
$1,800,000.00 (fully assembled product cost/month) x 1% (fallout) = $18,000/month.

That means over the course of a year, it is $216,000.00.

Another critical element needing to be realized is Company A’s bottom line with revenues. Let’s say the product retails for $1,450/unit. Not only is the 1% fallout costing Company A $216,000 a year in bad product costs, but it’s also important to look at the lost revenues.

Graph illustrating cost increases as problems found later in product lifecycle.

Company A retailed product: $1,450/unit
Company A manfacturing plan of assembled products: 2,000/month
Company A revenue on final product: $2,900,000.00/month

With the 1% product fallout as described above, this means:
$2,900,000.00 (assembled product revenue/month) x 1% (fallout) = $29,000.00/month revenue lost due to bad product.

Over the course of the year, Company A loses $348,000.00 in potential revenue.

Therefore, as the example illustrates above, having a good PCB design is vital, and the tools to test the design are just as important.

All in all, each additional percentage that is test accessible reduces the risk of product fallout, which:
-Reduces the amount of scrap pile that could form due to product recall
-Reduce time spent troubleshooting PCBs and identifying where the failure is occurring
-Make sure revenue goals are met with no missed opportunity
-Maintain positive brand recognition

For more information related to SSP’s DFT capabilities, please take a look at our test services page.

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