Tuesday, May 31, 2016
Publication Date: 08/1/2008
ARCHIVE >  August 2008 Issue >  Special Feature: Test and Measurement > 

Completely Automating PC Board Testing
Flying probes touch down on SMT test areas.

Automation is king in today's production environment — a fact dictated by the economics of today's manufacturing ethic. Efficiency and productivity levels are dictated by profitability, and that in turn is determined by product reliability. Since test is part of the process, it is only natural that PC board test solutions need to address the issue of providing a high level of test coverage.

High level diagnostics provide feedback, in real time, to manufacturing and/or design — all automated. Automation brings the added value of increased productivity, thus effectively lowering test costs; however, to be cost-effective in many of today's PC board manufacturing operations, characterized by a high number 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 do this, an automated solution must include the mechanical flexibility to allow the user to fully utilize it for high mix, low volume production as well as intuitive, streamlined software tools that optimize the time required for program preparation and system setup.

Automation has been available on traditional bed-of-nails test systems for many years, but companies are increasingly opting for flying probe test systems, due to the many advantages this technique offers. First of all there is no need to build and maintain a fixture for each product code, and this means a considerable savings in money and time. It eliminates the need to "fight" against the typical fixture problems (bad contacts, dirty pins, storage space, etc). Another advantage in terms of process optimization 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.

Suitable Test Methods
PC boards coming from the production line can basically have two principal types of defects: short circuits between normally adjacent tracks ("Shorts"), and interrupted tracks ("opens"). The PC board 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 of 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 PC boards is making it increasingly difficult to build fixtures that can access every test point. A flying probe system with sufficiently precise 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 used on the S24 VIP system from Seica. Basically, capacitive measurements are performed between each single PC board track and a reference point (that can be a single track or a ground plane). These capacitive values are saved in a database to be used as "golden" values during testing of subsequent PC boards. These "auto-learned" values undergo an automatic certification procedure and only the "certified" values go into the data base, which will be used to compare the values measured on the next PC board under test to determine whether or not the board is good or faulty.
Automated loading and unloading attachments for flying probe tester.

Using this test method, the total number of measurements performed on the PC board decreases with respect to the traditional resistive/continuity method, keeping test time down while maintaining the same level of fault coverage. From the test process point of view, this method has several advantages: PC board batch testing can start immediately from the first PC board without the need for time-consuming setup procedures, test time is drastically reduced, allowing higher productivity, there is no need to have a "golden" board, since the certification method builds its database of good values during the tests of the different PC boards in the batch, and the PC board 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 tests are then used only where necessary to pinpoint the location of any shorts or opens that have been found.

Simple Software
Test program generation must be fast and easy, in order to stay within the short timeframe allotted for setting up the process. Starting from the information in the CAD/CAM files of the PC board, the test system must be able to import these data, converting it into a format that can be used directly for creating the test program. In the world of PC boards, the standard format is IPC-D-356A; this format includes all of the information about the PC board (nets denomination, track path, pad positions and shapes, information about any embedded components) needed to generate the PC board test program automatically.

The data import procedure should be simple and straightforward; on the S24 VIP system, this procedure is integrated into a "process oriented" environment which guides the operator through only three simple steps, which means the first PC board is ready to be tested in a matter of minutes.

(1)Preparation. The Test Wizard proposes the window for importing the CAM data, which also enables the user to define some test parameters which define the shorts and opens threshold values. The software then generates the test program automatically, also providing a statistics report, showing all of the nets and pads which are taken into account during the test.

(2)Verify. At this point, the user must physically place the PC board in the test system. Because of the data import procedure, the PC board information (layout, tracks, pads, etc) are graphically displayed.

The operator must choose two reference points (also called "fiducially") using the integrated camera to "learn" their positions. This process "aligns" the PC board and the test system; in fact, once this process is finished, the system can automatically calculate the correct physical coordinates of all points on the PC board.

(3)Test. At this point, the user can run the test of the PC board. The test results in a Pass or Fail message, and any defects found will be indicated in the test report. On the S24 VIP system, the Repair Station environment helps the operator to locate the position of the failed net on the PC board, and includes statistics, functions, and procedures for the retest of the failures. A graphical display shows the operator the implicated tracks of the failed board with the relative internal layer, and identifies the precise location of the defect with a flashing cursor.

Completely Automated Solution
Up till now, we have described possible solutions for minimizing the time and effort required to get a PC board test program up and ready on a test system.

To have 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 longer with respect to most of the other manufacturing processes, the most commonly used configuration for test is an "island" type, with external PC board magazine-loaders connected to the internal loader of the test system. Because of 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 system.

The Seica Multi Board Type Handler (MTBH), connected to the S24 VIP system, provides this type of capability; the loader can handle up to 100 PC boards 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 PC board, automatically adjusting the rails, loading the PC board and running the appropriate test program. After each PC board is tested, it is unloaded automatically and conveyed to separate storage shelves, according to the Pass/fail test result, and the test data are automatically saved to a file for statistics and repair. Optional "markers" can also be provided to automatically stamp each PC board with the test result. The use of this type of automation solution reduces operator intervention to a minimum. Thus fully automating PC board testing is entirely feasible, but naturally any kind of automation requires a higher initial investment. This higher cost is invariably offset by the higher productivity and lower operating costs in the medium/long term. On average, the 20 to 25 percent higher initial cost is recovered within the first year, or even sooner, in a 24/7 manufacturing environment.

The automated solution we have described here can also be used to add a "virtual" second or third shift for PC board test, since even those companies that operate on an 8-hour basis can take advantage of the off hours to test up to 100 boards, without the presence of an operator.

Contact: Seica Inc., 50A Northwestern Drive, Suite 10, Salem, NH 03079 603-890-6002 fax: 603-890-6003 Web:

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