Reading time ( words)
Changes in manufacturing processes and economics within companies, along with the short ramp times from design to end-product manufacturing, increasingly justify the use of automation to achieve higher levels of efficiency and productivity. Since testing is part of the manufacturing process, PCB test solutions equally must provide test coverage and diagnostics, as well as real-time feedback to manufacturing and/or design, in an automated way. Walter Gueli, Seica, examines the possibilities to fully extend the automated production choice to the world of PCB test.
Automation brings the added value of increased productivity, effectively lowering test costs. However, to be cost effective in many of today’s PCB manufacturing operations — high-mix situations characterized by a large range of different part numbers and low- to medium-volume production runs — any automated solution must have a high degree of flexibility and short set-up time. This is achievable if the automated solution includes the mechanical flexibility to allow the user to fully utilize it for low-volume/high-mix (LVHM) production as well as intuitive, streamlined software tools that optimize the time required for program preparation and system set up.
The goal, in regard to engineering a process to integrate an automated line, is to increase manufacturing efficiency, maintain high quality, and reduce costs. These three improvements allow manufacturers to compete in the global market. This choice must also be compatible with today’s compressed new product introduction (NPI) cycles, which require a fully functional manufacturing process to be up and running in extremely short times. Generally speaking, the last step in electronics manufacturing is test. We will examine here the possibilities to fully extend the automated production methodology to the world of PCB test.
Testing Solution on the Fly
Automation has been available on traditional bed of nails test systems for a long time, but increasingly, companies are opting for flying probe test systems. What are the advantages? There is no need to build and maintain a fixture for each product code, which means considerable money and time savings, not least by eliminating the need to fight typical text fixture problems (bad contacts, dirty pins, storage space, etc). Another advantage in terms of process optimization is that flying probers have virtually no system set-up time; to change to a different product code, the operator simply selects the required test program from the list of existing programs or, in the case of a new product, generates a new test program.
Suitable Test Methods for Typical PCB Faults
Shorts and opens. PCBs coming from the manufacturing line generally have two main types of defects: short circuits between (normally adjacent) tracks — commonly called shorts — and interrupted tracks — commonly called opens. The PCB test must therefore be aimed at detecting and diagnosing these shorts and opens.
The most exhaustive test method includes resistive measurements between one single track and all the others to detect shorts, and continuity measurements on all the nets to detect opens. This is the approach commonly used on bed of nails systems, but the growing density and miniaturization of today’s PCBs is making it increasingly difficult to build fixtures that can access every test point. A flying prober with sufficiently accurate probing capability can solve the accessibility problem, but test time can become an issue on boards with a high number of nets.
An alternative test approach is specifically designed for a flying probe test.* Capacitive measurements are performed between each single PCB track and a reference point (that can be either a single track or a ground plane). These capacitive values are stored into a data base to be used as golden/control values during the subsequent tests of PCBs. If these automatically learned values undergo an automatic certification procedure, only the certified values will go into the database. This database can be used to compare the values measured on the next PCB under test to determine whether the board is good or faulty.
With this test method, the total number of measurements performed on the PCB decreases in comparison to the traditional resistive/continuity method previously described, keeping test time down while maintaining fault coverage. From the point of view of the test process, this method presents several advantages. PCB batch testing can start immediately from the first PCB with no time-consuming set-up procedures. Test time is drastically reduced, allowing higher productivity. No golden board is required, since the certification method builds its database of good values during the tests of the different PCBs in the batch. The PCB pad or test point is physically contacted only once by the probes, avoiding the mechanical stress of multiple probe hits required for a complete resistive/continuity test. These traditional types of tests are then used only where necessary, to pinpoint the location of any shorts or opens defects found.
Kelvin and Barrel. To satisfy the test requirements of the new PCB generations, a higher quality standard is required. The Kelvin and Barrel tests are the answer. Those tests are based on 4 wire measurements of through-hole/via resistance (Barrel) and track resistance (Kelvin) with values lower then 1m?.
With Kelvin and Barrel tests, the PCB manufacturer is able to guarantee a high quality level.
Thanks to new generations of test probes and mechanical architectures on automated line test systems, the probers can perform any type of tests (Kelvin and Barrel tests included) without changing probes, reducing cost. Modern flying probers may have 4 or 8 completely independent mobile probes, probing top- and under-side PCB surfaces. Linear guides also enable high speed with the maximum accuracy and measurement repeatability.
Depending on test volumes, a 4- or 8-probe system will be appropriate. Use a 4-probe flying prober for low and middle volumes; 8-probe testers suit middle and high volumes.
A Completely Automated Solution
In this article, I have described possible solutions to minimize the time and effort required to get a PCB test program up and ready on a test system. To reach a fully automated test solution, it is necessary to choose a test system that can work in line, with automatic loading/unloading PCBs. Since test times are generally longer than most of the other manufacturing processes (print, pick-and-place, etc.), the most common testing configuration is an “island” type, with external PCB magazine-loaders connected to the internal loader of the test system. Due to the growing requirement to handle low-volume, high-mix production jobs, the optimum solution should accommodate multiple product types and quantities, to take full advantage of the 24/7 operating capability of an automated solution. Handlers should have flexibility for multiple PCBs and PCB types, as well as a governing software system to control test sequence.
Figure 3. A detail of the loading system.
With good board handler set-up (Figure 3), the automated test system can proceed to test each PCB, automatically adjusting the rails, loading the PCB and running the appropriate test program. After each PCB is tested, it will be unloaded automatically and conveyed to separate storage shelves, according to the Pass/Fail test result, and the test data automatically saved to a file for statistics and repair guidance. With this type of automation, operator intervention diminishes to a minimum.
* Measurements described here were performed on Seica S240/S280 test systems.
Walter Gueli is product manager at Seica.
Figure 4. Test system.
SMT, April 2010