The changes in manufacturing processes and economic realities in many different companies, as well as the short times needed to go from design to manufacturing of the end product, increasingly justify the use of automation to achieve the necessary levels of efficiency and productivity.
Since testing is part of the process, it is natural that PCB test solutions need to address the issue of providing a high level of test coverage and diagnostics, while providing feedback, in real-time, to manufacturing and /or design, in an automated way. Automation brings the added value of increased productivity, thus effectively lowering test costs. However, to be cost effective in many of today’s PCB manufacturing operations, characterised by a high amount of different part numbers and low to medium production runs, any automated solution must have a very high degree of flexibility and a very low setup time. In order to achieve this, an automated solution must include the mechanical flexibility to allow the user to fully utilise it for high-mix, low-volume production as well as intuitive, streamlined software tools that optimise the time required for program preparation and system setup.
The goal, in regard to engineering a process to integrate an automated line, is to increase manufacturing efficiency, maintain high quality and reduce costs in order to compete in the global market. This choice must also be compatible with today’s compressed NPI (new product introduction) 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 dedicated to testing. This article examines the possibilities to fully extend the ‘automated’ choice to the world of PCB test.
An advantageous solution ‘on the fly’
Of course automation has been available on traditional bed of nails test systems for quite a long time, but more and more companies are opting for flying probe test systems, due to the many advantages this technique presents. First of all there is no need to build and maintain a fixture for each product code, and this means considerable savings in money and time, not least by eliminating the need to battle the typical fixture problems (bad contacts, dirty pins, storage space, etc). Another advantage in terms of process optimisation is that there is virtually no system setup 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.
The suitable test methods for typical PCB faults
PCBs coming from the manufacturing line can basically 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 two categories of faults. 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 miniaturisation of today’s PCBs is making it increasingly difficult to build fixtures which 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, specifically designed for a flying probe test, is present, for example, on the Seica S240/S280 test systems. Basically, 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 in a database to be used as ‘golden’ values during the test of subsequent PCBs. These auto-learned values undergo an automatic certification procedure and only the certified values go into the database, which will 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 the same level of 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 need for time consuming setup procedures, and test time is 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, and 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 last types of tests are then used only where necessary to pinpoint the location of any shorts or open defects found.
In order to satisfy the new PCB generation, a higher quality standard is required.
In addition to the standard tests oriented to detect a possible open on a track or a possible short between tracks, new and special tests are requested. The Kelvin and Barrel tests are the answer to this new requirement. These tests are based on 4-wire measurements used to measure through-hole/vias resistance (Barrel) and track resistance (Kelvin) with values lower then 1 m. Utilising those tests, the PCB manufacturer is able to guarantee the high standard level of the product.
Thanks to the new probe generation and mechanical architecture, the Seica BBT automated line systems can perform any type of tests (Kelvin and Barrel tests included) with the same probes. The systems are equipped with four or eight (S240 and S280) completely independent mobile probes, 4/2 probes on each board side. Thanks to the third generation of linear guide, they are able to achieve very high speed with maximum accuracy and measurement repeatability. The S240 flying probe system is indicated for low and middle volumes, whereas the S280 systems are suitable for middle and high volumes.
A completely automated solution
In order to reach a fully automated test solution, it is necessary to choose a test system that can work in line, with automatic loading/unloading capability. Since the time required for test is generally long with respect to most of the other manufacturing processes, the most commonly used configuration for testing 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 types of production, the optimum solution should be able to accommodate multiple product types and quantities, to take full advantage of the 24/7 operating capability of an automated solution.
Seica’s multiboard type handler, connected to the S240/S280 systems (four and eight heads), provides this type of capability: the loader can host up to 150 PCBs at a time, whether they are all the same or all different product types. A simple software menu allows the user to set up the test sequence, and then the test system will proceed to test each PCB, automatically adjusting the rails, loading the PCB and running the appropriate test program. After each PCB is tested, it is unloaded automatically and conveyed to separate storage shelves, according to the pass/fail test result, and the test data is automatically saved to a file for statistics and repair. The use of this type of automation solution reduces operator intervention to a minimum.
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