Lowering the cost of test with Boundary Scan
27 June 2016
The cost of manufacturing printed circuit board assemblies (PCBAs) has always been a concern for original equipment manufacturers (OEM) and contract manufacturers (CM).
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This is one of the reasons why electronics manufacturing has been shifting from one region to another region to stay competitive in this challenging environment.
An important part of the manufacturing process is testing the PCBA. Figure 1 shows the different stages of PCBA manufacturing, starting from the bare printed circuit board undergoing solder pasting, after which components are placed one by one by an SMT machine at high speed. The PCBA then goes through the oven to melt the solder paste and create a connection between components and the PCB. Each stage of these manufacturing processes is an opportunity where a defect might be introduced by the process itself, by human error or by the equipment. One of the final stages of the PCBA fabrication is testing, to ensure that all components are correct and properly connected to the PCB. This is where the challenge starts, with a series of tests conducted on the PCBA to ensure defects are captured and corrected before the PCBA goes to another stage of the manufacturing stage.
The common PCBA testing stages consist of the following:
1. In Circuit test (ICT) – Test the PCBA for opens and shorts, component value (resistor, capacitor, inductor, diode, transistor and FET), powered tests such as measuring voltage on board and checking the functionality of individual digital components. Common defects arising from manufacturing faults include open, shorts, and wrong components.
2. Functional Test (FT) – PCBA is powered to enable checking of the functionality of the board. The defects are presented in a block of functions.
3. Repair Station (ICT and FT) – To repair the PCBA that failed during ICT and FT stage.
Design for test - Introduce boundary scan test at prototype phase
Boundary scan or 1149.1 is an IEEE Standard that defines the test access port and boundary scan architecture of digital integrated circuits to allowed testing of devices in a PCBA. A boundary scan device is designed with shift registers known as boundary scan cells that are placed between pins of the devices and the internal logic (see Figure 2). These boundary scan cells allow control and observation of what happens at each input and output pin of the boundary device. When these device pins are connected to other boundary scan devices, it will allow connectivity testing of each device.
Boundary scan has become an important limited access solution for PCBAs. Boundary scan usage has also expanded to include testing of non-boundary scan digital devices such as DDR and programming digital devices such as flash, EEPROM and serial peripheral interface (SPI) devices. Boundary scan also has the capability to execute other tests, as defined in the BSDL (Boundary Scan Description Language), including private instructions which support internal functions of a boundary scan device, such as built-in self-test (BIST).
The success of boundary scan lies with the proper design of the board and verification of boundary scan at the early stage of the board design to ensure success during production implementation. Hence the PCBA test strategy should start at the design stage of the board to ensure that maximum coverage is achieved during testing. Boundary scan can be used as part of the test strategy from the prototype stage, right to new product introduction (NPI) and production run stages to achieve the highest test coverage at every stage of the PCBA process, and at the same time lower the cost of test implementation.
Boundary Scan at prototype stage
It will be very cost-effective if manufacturers can consider if their PCBA has proper design for boundary scan tests during the prototype design stage, before the board is actually assembled. This can help assure the board designer that the PCBA will have maximum test coverage, without worrying for possible structural defects of the board. This will result in a shorter time to market for the PCBA to move to the next stages of the product cycle - NPI and Production.
Another advantage of using boundary scan during the prototype phase is that the designer will be able to identify what nets of the board will not require testpoints. He can also determine early on which tests require boundary scan, while assigning other testpoints into nets that will only be tested during ICT.
Boundary Scan during NPI
Successful boundary scan implementation during prototype build will ensure the success of boundary scan at the next stage, which is the NPI phase. During NPI build, ICT is typically used as part of the test strategy, during which the ICT program will be developed and the ICT fixture will be built. Boundary scan implementation at this stage will have the following advantages:
1. Since the boundary scan test program is already created and debugged during the prototype phase, the ICT development time will be shorter.
2. During prototype, the nets that need to be tested with boundary scan have already been identified and separated from those that need ICT. This will mean fewer testpoints needing ICT - translating to lower cost of ICT due to fewer analogue/digital cards needed.
3. The cost of the ICT fixture is lowered since the PCBA has fewer testpoints.
4. The overall cost of ICT program development is cheaper since the boundary scan test program is already created during the prototype stage.
Boundary Scan during production stage
The same boundary scan test programs that are created during the prototype phase and integrated in ICT during the NPI stage can still be used in various stations during volume production (see figure 4).
1. ICT station – The boundary scan test developed during Prototype and NPI will be integrated in the ICT.
2. ICT repair station – The same boundary scan test can be used at the ICT repair station.
3. FT station – The same boundary scan test can be used and integrated during functional testing of the PCBA.
4. FT repair station – The same boundary scan test can be used at the FT repair station.
Boundary scan implementation across all the product manufacturing and testing stages can lower the cost of test as the same hardware and software can be reused while simultaneously ensuring higher test coverage. The reuse of boundary scan test programs across test stations also ensures the quality of test is maintained at all the test stations. This will help the production operator and technician become familiar with the defects being discovered at the various test stations, and help ease repair at all stages of production testing.
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