Printed Circuit Board Assembly and Subassembly test
Prototype Verification |
Structural Manufacturing Test
Automated Optical Inspection |
Automated X-Ray Inspection
Field service
The diagram on the right shows a typical electronics manufacturing flow, where on the left side we start with
the screen printing process for the carrier (typically a printed circuit board) and on the right side we finish
the manufacturing process with final assembly test before shipping the end product.
In-between we typically have test steps at various stages of the manufacturing process.
For example, Automated Optical Inspection (or AOI) is widely used after component placement and before reflow soldering.
Often there is another AOI or even X-Ray Inspection step after reflow. The goal is to sort out bad units before passing them on to the next stage and to rework them immediately, if possible.
Since Inspection methods don't verify the electrical features of the unit under test, most test strategies include at least one electrical test methodology focused on manufacturing defects such as opens and shorts prior to running a functional test. Examples are Flying Probe Testers, In-Circuit Testers, Manufacturing Defect Analyzers, or a combination of those. These types of testers often times can provide much better diagnostics of manufacturing defects compared to Functional Test. Again, we would want to rework any faulty UUTs prior to spending test time on functional test equipment.
Practically all manufacturing test strategies include at least one functional test stage to verify whether the product functions per specification prior to shipment to the customer. None of these mentioned test methodologies is perfect, which is the reason why there is typically a combination of these test methods employed in electronics manufacturing.
This table compares Automated Optical Inspection, Automated X-Ray Inspection, In-Circuit Test, Flying Probe Test, Functional Test, and JTAG / boundary-scan test. We have listed a number of different types of faults, classified in Soldering Defect, Placement Errors, and Electrical Defects.Visible shorts, for example, can be detected by all six test methodologies. One may not want to use X-Ray inspection to look for this type of defect, though, since X-Ray equipment is expensive and X-Ray tests are typically slow.
For hidden shorts, on the other hand, X-Ray is a good inspection methodology, whereas AOI obviously cannot see such hidden shorts. In-Circuit Test, Flying Probe Test, Functional Test, and JTAG / boundary scan, all can detect hidden shorts because they actually test the electrical properties of the Unit Under Test, rather than just visually inspecting the UUT.
Visible Opens, again, can generally be detected by all six test methods, while hidden Opens cannot be detected by AOI equipment and may be difficult to detect with X-Ray equipment. Inspection equipment is good, though, for analyzing the quality of solder joints, whereas the other four test methods compared here cannot say much if anything about solder joint quality.
Moving on to Placement Errors, missing components can generally be detected by all six test methodologies, with some exceptions. A wrong component can typically be detected by the electrical test methodologies, but not by X-Ray inspection. AOI can detect a wrong component only if there there is a visually recognizable feature that differentiates if from the correct component and the AOI test is capable of picking up this difference. Problems related to component orientation, on the other hand, are typically detectable by AOI, as well as by the electrical test methods. X-Ray inspection, however, usually cannot detect orientation problems.
Misplaced or misaligned components whose device pins still make contact with the pad on the PCB can cause problems later during shipping and handling or once the product is deployed by the end user. Such alignment issues typically can be picked up only by inspection equipment, but not by electrical test methods.
The third category of defects discussed here, Electrical Defects, per definition can not be detected by inspection equipment. The four electrical test methods vary in their capabilities to detect these types of defects. Defective components and defective PCBs can typically be identified by all four electrical test methods, while ESD related problems, circuit design related problems, and software related faults are typically detectable only by Functional Test equipment and partially by JTAG / boundary scan equipment.
Each of the six test methods discussed here has its merits. AOI, for example, can be used very early in the manufacturing process, keeping the rework cost low. The downside of AOI, however, is the lack of verification of the UUT's electrical properties.
Automated X-Ray Inspection has the benefit of using, well ... X-Rays - it can be used to inspect hidden solder joints that otherwise may be difficult to verify. AXI equipment can be very expensive, though, and test execution time can be lengthy. Also, X-Ray equipment does not verify any electrical properties of the UUT.
The main benefits of In-Circuit Test are the speedy test execution and the thorough test of board-level electrical properties, as long as sufficient test access is provided. And that test access can be quite a handicap these days - the bed-of nail test fixtures ICT requires can become very expensive and the number of circuit nodes that are accessible with nail probes may be rather limited on complex PCBs, reducing the test coverage achievable with ICT.
Flying probe Tests don't use a bed-of nail fixture like ICT, rather they have a number of independently moving probe heads that can be used to contact accessible circuit nodes. The types of tests a Flying probe Tester can execute is very similar or even identical to an In-Circuit Tester, however, it does so at a much slower speed, since the circuit nodes are contacted sequentially, rather than in parallel.
One might think Functional Test is perfect when looking at this table, since it can detect most of the listed defects. However, test development is usually manual and time consuming, diagnostic capabilities can be rather limited, test execution time can be length, depending on UUT complexity, and functional test equipment can be expensive.
JTAG / boundary scan has the benefit of being able to be used throughout the product life cycle and potentially without any fixturing requirements.
JTAG / boundary scan tests can even be embedded on a board or in a system, for remote tests and/or to be part of power-on self tests. JTAG / boundary scan focuses on digital circuitry. However, boundary scan cluster tests, integrations with Functional Test, and new and upcoming IEEE standards can overcome this limitation to a large extend.
So, in summary we can say that none of these test methodologies is perfect and none of them should be used solely by itself before shipping the end product to a customer.
GOEPEL Electronics offers JTAG / boundary scan solutions (SYSTEM CASCON software, SCANFLEX and SCANBOOSTER hardware, as well as a variety of accessories and integration packages), Automated Optical Inspection systems (Opticon), high-speed inline 3D Automated X-ray Inspection systems (Opticon X-Line), and various software and hardware tools for functional test application.
Use the following links to learn more about specific board level test applications or e-mail us for any inquiries you may have.
Prototype Verification |
Structural Manufacturing Test |
Automated Optical Inspection |
Automated X-Ray Inspection |
Field service