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History of Inner-layer Testing
December 31, 1969 |Estimated reading time: 5 minutes
With the advent of high-speed flying probe testers, prototype PCB manufacturers are finding that with electrically testing inner layers, something old is new again. During the 1990s, the flying probe tester found its niche. This article outlines the use of flying probe testers for inner-layer testing of PCBs.
By Brian Marcinak and Gary Stoffer
In the mid 1980s, there was a move toward electrically testing inner layers. During the mid-80s, electrical testing of finished PCBs was an accepted, if not expected, part of the production process. The advent of universal-grid testing machines made this all possible in the 1980s. Still, the cost of inner-layer test fixtures, the number of these test fixtures, the high probe count required, and the tight center probing of vias made this all but impossible for most products. With the advent of automatic optical inspection (AOI), electrical testing of inner layers was quickly forgotten. Some printed circuit manufacturers who had initiated entire departments dedicated to inner-layer test closed those departments and moved test equipment to final inspection.
During the 1990s, the flying probe tester found its niche. While still slow compared to grid testers, flying probe testers did not require a test fixture. The flying probe could also probe small targets on very close centers. These capabilities removed two obstacles in testing inner layers. However, test speed and product handling issues remained.
In many cases, manufacturers have no reason to electrically test inner layers. Most have success in test departments testing thin products (i.e., flex circuits) with custom board holders and product-tensioning frames. Although test times were slow on the flying probe for thin products, they could still be tested. This accompanies the problem of solving escape issues (shorts and opens on inner layers) in AOI. To do so, a PCB prototyping manufacturer* performed a series of experiments. The first test was done to confirm that AOI was the process point in which escapes were occurring. Unannounced to any of the production staff, a random batch of inner layers that had successfully gone through AOI was taken to the test department. Test programs were loaded into a 16-head flying probe test system,** and all inner layers were tested. The test time was relatively about 10 minutes to test both sides of a fairly large and complex inner layer.
Figure 1. Machine with custom board holders and product-tensioning frames.
Test results confirmed that opens and shorts were passing through AOI machines. The failed inner layers were returned to the AOI department and processed on the original AOI machine with the original setup. The machine found all faults and some false errors. These results showed that errors were subjective - it was not the machine that was the issue, rather the stress AOI operators are under to process many jobs quickly. Adding to this, fatigue incurred when analyzing several false errors while trying to isolate real errors. Given the number of layers an operator must inspect daily, a user may change the tolerance of the AOI unit. Relaxing the tolerances reduces the amount of inspection per layer; decreasing overall time. A more forgiving tolerance will yield less potential faults that an operator must decipher The biggest flaw in the latter process is that a PCB shop will open itself to faults getting through the AOI department because they are not checked. All these decisions are left up to the operators. The conclusion is that the operator must be removed from the fault-decision process.
Electrically testing inner layers would work to remove the operator from the decision process if test times were faster and handling was comparable to an AOI machine. A vacuum frame for a 16-head flying probe test system** was developed (Figures 1 and 2). Previous benchmarks proved that most inner layers could be tested in 30-60 sec. The worst case seemed to take about two minutes to test the most complex inner layers. Product load and setup times with the optical-scanning (product alignment) capability of the flying probe tester were comparable to that of common AOI machines. Given there were few false errors from electrical testing and fast verification times, the flying probe tester proved faster than AOI.
Figure 2. Flying probe test system features 16 heads.**
The testing solution becomes a suitable choice once obstacles to inner-layer testing are overcome. This solution helps flying probe test departments run smoothly, eliminating reliability and ease-of-use issues. Flying probe testers also eliminate the decision-making process for machine operators, allowing the same staff to remain in the electrical test area as in the inner-layer inspection area. This results in more depth and less cross-training. The verification process is the same in final and inner-layer tests. There is no need for operators to specialize in one particular area.
Conclusion
With obstacles to electrical testing of inner layers overcome with the advent of high-speed and user-friendly flying probe testers, this method has become a way to eliminate inner-layer shorts and opens from many manufacturing processes. At several locations, some inner layers are inspected electrically, while other products are both electrically tested and tested using AOI for specific issues such as global-line width and spacing violations. Other products that require full AOI, such as controlled impedance, are electrically and optically inspected. Electrical testing of inner layers for shorts and opens makes it easy to introduce the test for resistance and capacitance testing of inner layers (i.e., buried resistors and capacitor testing). This solution can help to eliminate inner-layer shorts and opens from the manufacturing process, resulting in improved yields, fewer job remakes, and better price points for customers.
To stay competitive in the PCB industry it is critical that all processes bolsters the manufacturing of a reliable PCB. Inner-layer flying probing allows shorts and opens to be caught early. It also allows time and energy to be focused on ensuring the electrical integrity of the PCB prior to final lamination, without additional staff. Inner-layer testing results in improved yields, fewer job remakes, and a better price for customers.
*Sierra Proto Express, Sunnyvale, Calif.
**ATG A6-16, Everett Charles Technologies (ECT), Pomona, Calif.
***ECT, ATG group, Pomona, Calif.
Brian Marcinak, ET supervisor, Sierra Proto Express, may be contacted at (408) 328-4040. Gary Stoffer, manager, Everett Charles Technologies, may be contacted at (909) 625-9348; e-mail: stoffeg@ectinfo.com.