A board has passed through the final assembly process and culminated in a finalized, realized product that is likely months of combined design from engineering through manufacturing. While this stage represents a significant milestone, there is still apprehension about whether the board will pass through quality assurance. In reality, the board has already moved through some critical checkpoints to reach this stage. However, there are still numerous tests before a board is considered ready to ship.
PCB testing methods come in many different flavors and applications. Considering the amount of processing required to transform supplier materials into a fully functional piece of electronics, this may be hardly surprising. Still, it is worthwhile to understand the common elements underlining design testability and the explanation and justification for the multiple testing methods designers encounter.
Testability Forms the Basis of Methodology
As with any process involving PCBs, testing is not a simple choice nor a universal evaluation. Testing considerations will be essential to a design team’s evaluation from quality and cost-efficiency standpoints. Based on factors such as component density, material flexure, production quality, and
more, certain tests may gain or lose effectiveness in diagnosing manufacturing errors. Design teams can weigh various aspects to ensure a thorough approach to valid testing, including:
- Centralizing input/output and test points on a single edge connector for speed of testing. Additionally, tying test points into the edge connector can help ease test functionality without needing more elaborate in-circuit testing styles.
- Decoupling is not only a concern for integrated circuits’ power/ground return pathing – ensure capacitors are placed towards the extents of the board where necessary to avoid excessive current loops that can reduce responsiveness and cause EMI issues.
- Ensuring signal outputs are never tied together. Tie I/O pins high or low as necessary according to design needs and manufacturer’s documentation to avoid floating logic that can result in unreliable and erratic performance; a resistive pull-up can minimize noise sensitivity.
- Simplifying logic as much as possible reduces cost and the potential for masked errors.
- Providing a secondary override to logic evaluators (LEDs, displays, etc.) enables an easy method for determining incorrect operation via component or logic.
Testing form will follow function: design teams must consider how their board will interface with different test modules. Generally, redundancy and ease of access are preferred to allow testers ample ability to literally and figuratively probe the board from various angles. Simple design choices such as enlarged test point pads (to draw the eye and provide space for probing) and increased font sizes on pin one or polarity indicators will also aid testers as they work through the arduous task of validation. Realize that testers, as much as anyone on the design team, are under immense pressure to catch any issues before the boards continue through manufacturing or enter customers’ hands.
PCB Testing Methods for Electrical Continuity
Now that the design team has properly incorporated testability into the design, PCB testing methods must be determined. First will be verifying the fabrication process, the lengthier and less reversible of the two overarching divisions within manufacturing. Electrical testing will determine whether the netlist has been accurately translated and recreated in 3D space. Bare board testing will be required to accomplish this, though that term is nebulous and encompasses many options open to discussion between the design team and manufacturing shop. Some of the methods, and their uses, follow:
- Resistive Continuity/Isolation Testing: Probing a net’s resistance to determine whether it’s below a maximum value or above a minimum value. Both styles are compatible with the major in-circuit test mechanisms (bed-of-nails and flying probe). However, the latter may encounter issues cooperating with an Isolation test as the configuration is looking for shorts instead of opens.
- Insulation Resistance: A relatively straightforward test that applies a DC voltage to the insulation along with a known current to determine the average resistance throughout the media.
- Signature Comparison/Indirect Isolation: Uses additional network characteristics such as capacitance and RF values for fault detection. Errors detected indirectly will undergo further resistive testing to determine the specific type of anomaly (e.g., open or short).
Further electrical tests ensue, which adequately stress the board above normal operating conditions to determine where the breakdown occurs. Assuming failure occurs beyond max parameters established by the datasheet, the board is fit to proceed to additional manufacturing.
Hardware and Software Approaches to Assembly Verification
While assembly is generally a much less rigorous process than fabrication, testing can become challenging due to reduced access to the top and bottom layers of the board. This hardship most notably manifests during in-circuit testing, one of two major assembly-level quality tests. In-circuit testing utilizes a bed-of-nails fixture to sequentially access all of the components on the board via test points. On the other hand, functionality testing accesses the board via an edge or other off-board connector, stimulates inputs, and evaluates outputs to determine whether the two match. This step effectively proves the validity of the schematic, board, and manufacturing levels of design in a single pass.
However, with increasing design density and shrinking pitches, the future of in-circuit testing is on somewhat shaky ground. A new method that can quickly verify assembly while simulating runtime conditions is needed: boundary scan testing. Boundary scan testing is built into the design at the component level, leaving designers to take advantage of the embedded mode. Doing so requires the implementation of scan registers on I/O pins. These IC pins can connect with the greater I/O of the assembled board, forming a network where virtual in-circuit testing can be performed without the need for extensive equipment associated with the bed-of-nails fixture or flying probe machine. Instead, boundary scan testing is a programmable method where the designer can easily access the board and select circuits using the JTAG language.
Let your Contract Manufacturer Devise Tests to Maximize Validity
PCB testing methods run the gamut regarding deployability and viability, but DFM practices must also consider testing. When it comes time to build your design, the thought of adding another layer of complexity to the design stage may seem downright overwhelming – that’s where our team at VSE is poised to bring your product to fruition. VSE follows a simple mantra: as a team of engineers, we build electronics for customers to the highest possible standard. As your contract manufacturer, VSE will work with a team of highly experienced partners to maximize the potential of your board from beginning to end.
