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Home / About Us / Solutions / Low-volume PCB Testing / Del-Tron Solutions
Extremely Small Ball Slide Helps Enable Testing of
Low-Volume PCBs without Building Fixtures
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Probot Inc.
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A very small ball slide assembly helps make possible the testing of printed-circuit boards (PCBs) without building custom test fixtures, giving PCB manufacturers a faster and less expensive option for testing low-volume boards. The ball slide provides z-axis motion of test probes in a device known as a moving probe tester (MPT) from Probot Inc., Branford, Connecticut. The extremely low mass of the C-1 ball slide assembly from Del-Tron Precision, Inc., Bethel, Connecticut allowed Probot to move the probes fast enough to make the MPT method practical while keeping momentum low enough to create minimal witness marks. The small size of the C-1 also made it possible to let the two probes to operate in close proximity to each other, a factor is growing increasingly important as the grid size on PCBs is reduced.
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| For PCB manufacturers, the high cost and long lead time involved in building clamshell test fixtures is a major drawback on low-volume jobs, especially quick-turn prototypes. It can take up to 100 person-hours to build the clamshell fixtures required to test these boards on a bed-of-nails tester. The typical cost to build a clamshell fixture is about $500 for materials and programming, plus 25 cents per test point. |
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For a double-sided SMT multi-layer board with 6000 test points, for example, the cost would run somewhere in the neighborhood of $2000. Fine-pitch boards with feature sizes of 25 mils (635 µm) or less and double-sided SMT boards are particularly difficult to fixture. In many cases, the cost of building the fixture exceeded the production cost of the board. Probot Inc.'s MPT technology eliminates the need for custom test fixtures by using moving probes to travel to the appropriate test points on a PCB. The test can be generated from CAD/Gerber or net list data in about 30 minutes. The test takes longer than a bed-of-nails test to run because rather than having one probe per test point, the MPT uses two probes that must travel to each point. However, advances in the last few years have improved test speed by a factor of 10. A key advantage of MPT testers is their ability to test even very dense fine-line SMT and MCM products without the pin slips that are commonly experienced with bed-of-nail testers -- there are no pins so there cannot be any pin slips. Therefore, the accuracy of the test results is far better with an MPT. While the actual test time on an MPT is slower than on fixtured testers, the total cycle time for small runs is much faster because the fixture building time is zero, and the retest or verification time is zero. Probot Inc., a privately held company, produces a family of MPT systems capable of handling a variety of board sizes. The systems include user-friendly Windows-based software.
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Z-axis motion
In designing the MPT, Probot was particularly concerned with the motion of the test probes in the z axis, which brings them into contact with the PCB. Boards are frequently warped, so the z-axis position of the probe will vary across the board. When the probe makes contact, it must leave as small a witness mark as possible. A witness mark is a function of the momentum with which the probes hit the solder pad, which in turn is a function of the mass of the ball slide times its velocity. The challenge Probot faced was getting enough velocity to make this test method a good alternative to custom clamshell fixtures. These fixtures have one probe for every test point. They can close slowly to ensure minimal witness marks since they only have to close once. The probes on the MPT must touch each test point, so they must operate fast enough to make this approach feasible yet without so much momentum that the probes damage the pads or leave too great a witness mark.
Another consideration in the design of the MPT was the ability to bring the two probes in close proximity to each other. Although this was not a huge challenge when the company started building MPT systems 12 years ago, it has grown in importance with the increasingly tight grid spacing on PCBs. These days, the probes must come within .003 to .004 inches of each other when testing to determine whether neighboring nets are causing a short in the net of interest.
After examining a number of different linear motion devices, Probot engineers selected a C-1 Del-Tron ball slide assembly to provide the z axis motion. With dimensions of .75 long inch by .23 inch high by .38 inch wide, this is the smallest ball slide assembly on the market. Its small size and lightweight aluminum carriage translates into the smallest mass of any device available. The small mass reduces momentum while enabling the probes to move fast enough to contact thousands of test points in a reasonable amount of time. After programming and setup, which take approximately 30 minutes, a board with 6000 test points can be tested in half an hour or less. Even at that speed, however, momentum is low enough that witness marks are minimal.
The assembly's small size allows Probot's MPT to move the probes as close together as they need to be. The C-1's size is related to its extraordinarily low friction. Because it uses linear ball bearings, it has less friction to overcome and can therefore have a low mass. Much more commonly used rotary ball bearings have a higher coefficient of friction because the peripheral track is shorter on the inner race than on the outer race, causing the ball to skid on one or the other. Linear ball bearings run exactly equal distances on the pair of tracks in linear bearings, permitting the balls to run without friction, wear, or skidding at any preload. Steel shafts, ground over the entire length also reduce the coefficient of friction, which is typically 0.003 for Del-Tron ball slides.
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Help with assembly
The probes are attached to the C-1 ball slide assembly with two screws that attach to built-in holes to simplify installation. The mounting surfaces of the ball slide are machined flat and smooth, and parallel to each other and the line of motion. They must be mounted on smooth, flat supports that will not deflect under load.
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When Probot started building its first MPT systems, it seemed there was a problem with inconsistency in C-1 ball slides preload. Since preload is set at the Del-Tron factory and Probot was not changing it, Probot engineers called Del-Tron for help. Del-Tron was very helpful in solving the problem, which was due to the fact that the mounting surface on the MPT machine was not perfectly flat. Since the ball slide assembly is made of aluminum, it was conforming to the MPT machine and losing its adjustment. Once they improved the flatness of the mounting surface, the C-1 performed perfectly. Del-Tron ball slides are well suited to the MPT application because they require exceptionally low levels of maintenance. They are lightly lubricated during assembly, and are self-cleaning in normal service. Additional lubrication is not required, except for applications involving speeds above 1800 inches/min or continuous high loading. When used at the rated load capacity and moderate speeds, a life of 10 million inches of travel can be expected. The expected life at one half the rated load is 100 million inches.
The use of an MPT system on low- and medium-volume PCB jobs has helped Spectra, a contract PCB manufacturer based in Clarksburg, Maryland, reduce production costs on low volume (1-7 pieces) and very high tech work (sub 0.016" [406 µm] pitch) by an average of 20% compared to fixture-only testing. At the time it was purchased, the MPT cost $200,000. The machine operates 24 hours a day, five days a week. Expensing over five years yields a cost of just under $10 per hour. Assuming a $40 hourly rate including labor and overhead, it costs only $80 to test four typical boards. This provides a dramatic savings from the $2,000 required to build just one clamshell fixture. By passing these savings on to customers, Spectra has been able to achieve a competitive advantage. Lead time on low-volume jobs has also been significantly improved because the time needed to build a clamshell fixture often exceeded the time needed to build the boards.
Although the small C-1 ball slide assembly is not something the user is ever aware of, its operation plays an important role in the performance of an MPT system. Its extremely low mass is the critical component that allows the probes to contact PCBs at high enough speeds to make MPT practical, while keeping momentum low enough to leave only a very small witness mark on the pads.
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Del-Tron Precision began operations in 1974 supplying original equipment manufacturers with the world’s first commercially available subminiature ball slide. Since then, thousands of Del-Tron ball slides have been incorporated into medical analyzing and testing machines, semiconductor processing equipment, computer peripherals, assembly systems, scientific instruments and many other machines. Del-Tron’s modern corporate campus boasts highly automated computer controlled equipment and final inspection of 100% of all products has been Del-Tron’s policy since its inception. For more information, contact Del-Tron by phone at 800-245-5013, by fax at 203-778-2721 or by email: deltron@deltron.com. |
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