Flying Probe Testers and Boundary Scan

Today’s flying probe testers can give high structural test coverage, making them ideal for prototype board bring-up and low-volume manufacturing. But they can be darned slow. Can boundary scan help?

Flying Probe Testers (FPT) meet the “gold standard” for early prototype testing, because they don’t require an expensive fixture like traditional In-Circuit Testers (ICT). Especially given that one or more board spins may be needed during the early stages of a product’s lifecycle, the cost of re-doing the fixture can be cost-prohibitive. So FPT is a reasonably low-cost, get-started-quick way of verifying that a board has been assembled correctly.

Of course, once the design has stabilized and if the production volume is at all high, many companies migrate to ICT. Why is that? Because FPT can be pretty slow, and time means money. Although today’s models are much faster than yesteryear, it can take hours to test a high-node-count board.

As an example, a customer was using FPT on a board with over 6,000 nets. It was early prototype time, and their volume ramp-up quantities were uncertain, so they were naturally reluctant to spend $80,000+ on a fixture and ICT test program. For this size board, the FPT test took several hours. The board’s material Cost of Goods Sold (COGS) was $2,000, and adding in test time cost on a fully allocated basis of $200 took the gross margin COGS to $2,200; a hit of 10%. They were desperately looking for ways to reduce the test time, to improve the margin on their board production.

This is where boundary-scan test came in. By simply using the board’s JTAG infrastructure, they were able to eliminate overlapped digital fault coverage, thereby reducing the FPT test time from hours to minutes. One of the engineers calculated that the tester contribution to COGS went down from 10% to 2%. Can you guess who was the hero?