From test coupons and burn-in chambers to ICT and flying probes, effective factory testing is essential for delivering reliable products and maintaining process control.
Factory testing comes in numerous flavors. The goal is to ship products that work in the field. Failures erode gross margins, which in turn affect the company’s market perception. Good customer relationships depend on the timely delivery of products that meet the requirements, which is the definition of quality. IPC-TM-650, Test Methods Manual covers the test procedures in detail.
This column covers the following topics:
- Disposition of defective materials
- Statistical process control
- Stress testing power delivery networks
- Bed-of-nails and flying probe test fixtures.
On the factory floor, units come off the line in rapid succession. Testing is performed, although resources for troubleshooting may not always be available. Rejected units may be recycled or repurposed after the removal of higher-value components, depending on the industry and component mix.
When a unit fails, it may be sent to a caged area of the factory known as the material review board (MRB), where it remains until the next review meeting. MRB teams typically meet weekly, although critical issues may warrant an immediate review. Quality engineers, manufacturing engineers, planners and other stakeholders determine the appropriate disposition: use as is (UAI), return to vendor (RTV), rework or scrap.
Test coupons and solder samples. Impedance coupons and solder samples may be included as part of the qualification process. The impedance coupons contain transmission lines similar to those used on the board and are measured at frequencies representative of normal operation. The impedance of the bare board typically must fall within ±10% of the specified nominal value before assembly can proceed.
Solder samples are subjected to tests intended to identify warpage, measling, delamination and other defects associated with the soldering process. These requirements are typically specified in the purchase order. The purchase order supersedes information on the fabrication drawing, while local and state regulations ultimately govern the agreement. Some purchase orders also require a certificate of compliance. Electrical testing using an IPC-356 netlist is commonly certified in this manner.
Tests exist for nearly every aspect of a printed circuit board. Warpage can be measured using a granite table and go/no-go gauges. A light table can reveal internal board features, while x-ray inspection is used to evaluate hidden solder joints beneath BGA devices. Simple adhesion testing can determine whether solder mask and silkscreen materials are properly bonded to the board.
Statistical process control. The goal in manufacturing is to ship products with at least Three Sigma quality. That is where 99.7% of the products work right out of the box. This level of quality is enough to say that the process is in control. Satellites, submarines and other stuff may call for Six Sigma quality where 99.99966% of the articles meet the requirements. The nondestructive testing may be performed on all units, but more likely, a random sample is selected.
If one or more samples fail to meet the acceptable quality level (AQL), the entire lot is rejected. The number of inspection samples for the next lot may be increased. That’s also when destructive testing of the defective samples starts. The reliability lab will tear down some or all the units to find root causes and solutions.
It will probably not be economically or environmentally viable to put the whole batch into the dumpster. Instead, the units may be earmarked for a 60-day warranty rather than the usual year or two. The marginal lot would then end up with a reseller on Temu or perhaps a Dollar Store chain rather than a big-box retailer.
Behavior under less than ideal conditions. One of the key things to test, aside from the physical attributes, is whether the product can operate across the full range of acceptable voltage parameters. Every voltage domain has a minimum and a maximum input range. The standard +5V could range from 4.5V to 5.5V. The +/-10% may apply to 3.3V, 2.8V, 1.2V, and so on.
One phase of the test process would have the +5V cranked up to +5.5V while all other voltages are set to their minimum allowable values. Every possible combination of acceptable min and max power is tested. The “corner cases” may seem extreme, but they are emblematic of the methods used to evaluate electronics. More voltage domains equal more combinations to test.
While running at the various corners, look at the performance metrics. It could be an eye diagram or a Smith chart, whatever metric applies to the product. This type of stress testing will reveal any shortcomings in the power domain and signal integrity concerns.
Operating at high temperatures or cycling through temperature extremes will prematurely age electronics. A burn-in chamber will be used for this purpose. Sometimes the product goes full tilt until something breaks. A month-long burn-in at 80% of rated capacity is an example. If it survives that, then the “infant failures” are out of the way, granting the confidence necessary to ship the product.
Automated testing of printed circuit assemblies. Narrowing down the fault is sometimes done with in-circuit testing (ICT). A bed of nails is a type of fixture with spring-loaded pogo pins that align with a pattern of test points on the PCB. The least costly of these uses one side of the board for probing as many connections as possible.
Figure 1. An in-circuit test (ICT) fixture for simultaneous probing of both sides of the PCBA. (Source: Q1Test)
Placing test points on both sides will require a more costly clamshell fixture. Placing the test points close to one another is also a cost driver. It’s OK to be near low-profile components, while taller components will have more generous space. Of course, more test points are better when implementing a test fixture.
Figure 2. Nominal size and spacing for an ICT test point. (Source: Author)
In a perfect world, test points would be placed on a coarse grid and not clustered in a specific area. Each ICT probe will exert some force on the assembly. The result could be that the board flexes under the strain of dense test point placement.
Figure 3. Test point spacing for flying probe testing, 0.7mm pad on 1.4mm pitch. This is a column of JTAG pins. (Source: Author)
Personally, I use the design for assembly (DfA) tool, which allows me to assess a part’s proximity to other parts on a case-by-case basis. The DfA table has rows and columns to account for every part type. I give test points their own attribute, so they can cozy up to the short parts, give some room to the taller ones, and set a larger value for test point-to-test point spacing. That’s the trick to getting the lowest cost ICT fixture.
Figure 4. The secondary side is used for flying probe ICT and selecting components. (Source: Author)
Flying probe testing. An alternative to the expensive ICT fixture is a flying-probe machine. It works sort of like a pick-and-place machine. Instead of placing parts, it has multiple probe heads that “fly” around the board and take electrical measurements between pin pairs. These machines perform better if a number of gratuitous test points are in the power and ground planes scattered around the board.
Figure 5. An oversized courtyard at the intersection of the ATE values will single out test points for spacing rules around other test points. The category name is performed at the symbol level while the actual component spacing is constrained within the PCB editor. (Source: Author)
Once programmed, the flying head probes will run the test repeatedly. The time it takes will depend on how many pin pairs it must probe. Flying probe is generally a slower process than a bed-of-nails fixture. The benefit is they are much more agile when it comes to a board revision where a few test points were moved, added or subtracted.
Figure 6. The different values in the DfA table create specific guidance in terms of spacing. The test point can be nearer to the capacitor than to another test point. (Source: Author)
Wrapping It Up
Tests and measurements become more difficult as component density increases. Competition to create the smallest, most efficient form factor drives products that are dominated by the need to do more with less. A JTAG connector may be our only access, if even that. Designers of embedded electronics are pulled in several directions at once. Be flexible.
John Burkhert, Jr. is a principle PCB designer in retirement. For the past several years, he has been sharing what he has learned for the sake of helping fresh and ambitious PCB designers. The knowledge is passed along through stories and lessons learned from three decades of design, including the most basic one-layer board up to the high-reliability rigid-flex HDI designs for aerospace and military applications. John's well-earned free time is spent on a bike, or with a mic doing a karaoke jam.
