DFT solutions help designers place access points on a PCB to create optimum probe placement solutions that match manufacturing constraints.
Electrical test equipment can be grouped into four main categories: In-circuit Test (ICT); flying probe test (FPT); manufacturing defects analysis (MDA); and boundary scan test (BST). Electrical test requires physical access to create electrical contact to the PCB. Typical contact points include: test pads; bead probes; vias; through-hole pins; and component leads. Electrical test equipment strengths include the ability to measure component value, to perform powered-up analog and digital tests and to ensure there are no shorts or opens on the PCB. Its weaknesses include the inability to determine component placement accuracy, insufficient or missing solder and significant PCB real estate for contact points.
For ICT, MDA and BST, the typical requirements are as follows: one test point per net, all test points on the bottom of the PCB, and maximize test point size. FPT is typically done from the top side of the PCB; test points are preferred, however, probing component pads is acceptable. Regardless of the electrical test method used, the following techniques will improve both coverage and reliability:
- Provide ability to disable clocks. Allows testing at lower frequencies, that results in more reliable tests and reduced false failures.
- Add pull up/pull down resistors on tri-stateable ICs. Reduces noise during test; eliminates back-driving components to a desired state; requires an additional test point to control the state of the pin.
- Add tri-stateable buffers or resistor pairs in feedback loops. Increases test reliability and reduces noise.
- Correct configuration of BST Test Access Ports (TAP). Requires a test point on the first TDI to inject the test pattern and on the last TDO to read the output; daisy chain TDO to TDI on multiple devices. Place test points on TMS, TCK and TRST to control the devices.
ICT offers the broadest spectrum of electrical test capabilities, including shorts and opens tests, passive component measurements, active device tests and powered-up analog and digital tests. While quite powerful, ICT has the greatest access requirements, which consume a significant amount of PCB real estate. MDA and FPT have lower access requirements but have correspondingly lower capabilities. An MDA offers shorts and opens tests, active and passive device measurements and little else. FPT supports a similar set of capabilities as ICT but has limited analog and digital tests, as well as a significant increase in test time. BST has the smallest access requirements but is restricted to devices that support BST.
Maximizing electrical test requires that the manufacturer perform extremely accurate Design for Test (DFT) analysis. Leading DFT solutions accurately analyze and determine the access points on a PCB, create optimum probe placement solutions that honor manufacturing constraints and generate tester programs and fixture data.
Structural inspection equipment can be grouped into four main categories: Paste Automated Optical Inspection (AOI), pre-reflow AOI, post-reflow AOI, and Automated X-Ray Inspection (AXI). It does not require physical contact with the PCB. Structural inspection strengths include: ability to determine component placement accuracy; insufficient or missing solder; integrity of solder joints; and no additional PCB real estate requirements. Its weaknesses include the inability to determine component value, to perform powered testing and to validate electrical performance of the PCB.
The concept behind AOI is similar, regardless of where in the manufacturing processes it gets deployed. The PCB is digitally imaged, and then various algorithms are run against the image to determine conformance to the inspection criteria. Process characteristics such as 2D and 3D solder paste deposition, component placement accuracy, component orientation and labels are verified. AOI is limited to inspecting only what is visible; if components obscure each other or BGA solder joints require verification, then AOI provides no value. Minimizing component overlap is a key consideration in the design process which will facilitate AOI.
AXI is a complex and expensive structural inspection method, but it provides capabilities that neither electrical test nor AOI can. AXI has the ability to validate obscured solder joints and component placement issues, since it generates the inspection image using X-Rays. AXI is the best test or inspection method to ensure proper solder joints are made on the PCB, particularly on BGA devices. To facilitate AXI, designers should minimize placing components under BGAs on the opposite side of the PCB.
Regardless of the test or inspection method used in manufacturing, a little understanding at the design stage goes a long way toward maximizing the effectiveness of the manufacturing process.
PCD&FMatt Wuensch is business development manager, Systems Design Division, Mentor Graphics Corporation;
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