Friday, November 7, 2008

JTAG for Production Test

Applying JTAG for Production Test

Production test, utilizing traditional In-Circuit Testers that do not have JTAG features installed, experience similar problems that the product developer had and more:
  • Loss of Physical Access to fine pitch components such as SMTs and BGAs reduces Bed-of-Nails In-Circuit Testers (ICT) fault isolation.

  • Development of test fixtures for ICTs has become longer and more expensive.

  • Development of test procedures for ICTs has become longer and more expensive due to more complex Ics.

  • Designer is forced to bring out a large number of test points, which is in direct conflict with his goals to miniaturize the design.

  • In-system programming is inherently slow, inefficient, and expensive if done with an ICT.

  • Assembling boards with BGAs is difficult and subject to numerous defects such as solder smearing.
Figure 7 shows a typical production flow configuration that includes a JTAG tester that tests all the interconnects between the UUT digital components and performs in-circuit programming of all the CPLDs and Flash memories. Some test engineers complement the JTAG test with an ICT that requires simpler fixture primarily testing the analog components.

Figure 7 - Typical Production Flow Configuration

Following the ICT analog tests, a comprehensive at-speed functional test is performed before the product is shipped. However, in many cases, test engineers are skipping the ICT test and moving from JTAG interconnect test to a functional test that includes thorough testing of the analog portion of the product.

The following are major benefits in using JTAG test and ISP in production:

  • No need for test fixtures.

  • Integrates product development, production test, and device programming in one tool/system.

  • Engineering test and programming data is reused in Production.

  • Fast test procedure development.

  • Preproduction testing can start the next day when prototype is released to production.

  • Dramatically reduces inventory management – no pre-programmed parts eliminates device handling and ESD damage.

  • Eliminates or reduces ICT usage time – programming and screening.
Production test is an obvious area in which the use of JTAG yields tremendous returns. Automatic test program generation and fault diagnostics using JTAG software products and the lack of expensive fixturing requirements can make the entire test process very economical. For products that contain edge connectors and digital interfaces that are not visible from the JTAG chain, JTAG vendors offer a family of JTAG controllable I/Os that provide a low cost alternative to expensive digital pin electronics.


Unknown said...

I am new to JTAG, so pls excuse the simple question: I fully get JTAG for product development, but for production pcbs is it normal to continue to support the JTAG interface on a real product so newer software releases can be programmed in?

JTAG said...
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JTAG said...

Leaving permanent JTAG access to your product is very beneficial and in many ways similar to how it is done in one's home. Houses are built with maintenance pipes and external access valves where a plumber can easily and non-intrusively gain easy access to the pipes for checking conditions, fixing sewer blockage, etc., and without having to ever dismantle walls to gain access to pipes embedded inside the structure. JTAG is kind of the same as leaving an accessible TAP connector allows you to test the entire product without having to take it apart. You can also update firmware, test board interconnects, upload calibration data and other system parameters, etc. and even test the unit while it is operating as a product.