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Board-support Analysis During Component Placement
December 31, 1969 |Estimated reading time: 6 minutes
To examine the forces applied to components, this test compares nozzle forces of pick-and-place equipment and those applied during paste deposition while using full vs. near-full bottomside component support.
By Steve Beck, Richard Lieske and Ricky Bennett
Various board-support tooling systems are available for processing, including manually placed pins, automatic programmable arrays and custom-machined plates. Most are designed to avoid component contact via placing support pins in exposed real estate areas on the board's tooling contact side. However, as boards become more densely populated and support-pin placement possibilities diminish, "adequate" board support often poses difficult choices. In fact, when board designs require components to be mated from the top side to the bottom side (or when component density is too high to place tooling), sufficient board support may be impossible to provide.
Figure 1. Examples of boards commonly manufactured that are difficult or impossible to support during assembly with standard tooling.
To overcome the unavailability of board-support real estate, some full-support tooling systems have been introduced. One uses a pin matrix to provide compliant, gentle support across the board's entire bottom side, which may include previously placed components. In both laboratory and field testing, the system functioned effectively without damaging bottomside components such as quad flat packs (QFP), cylindrical parts, surface mount connectors or test pins, and other fine-point units. The system also accommodated variations in board surfaces without component damage while supporting the stencil at the edges and in large, open areas normally subject to coining.
Standard ToolingIn the industry, manual magnetic pin support, custom tooling plates, programmable magnetic pins and programmable pin arrays commonly are referred to as standard tooling. Such tooling is designed to support the product by contacting the open board areas. The pros and cons of these tooling methods, including relative cost considerations, are reviewed in Table 1.
Figure 3. Print force measurements employed sensitive devices on pins' tops and monitored pressures applied by squeegee passage.
Table 1 reveals that the areas of concern for standard tooling include: support generally is limited to open board areas, and any tooling pin misplacements can cause serious component damage. The latter is a major concern for boards that are heavily populated on the support side yet still require good support for components on the print side. Figure 1 illustrates typical assemblies manufactured that are very difficult, if not impossible, to support with standard tooling.
Compliant Tooling: Lab and Field TestsFully compliant tooling is designed to support the board and tooling-side components while providing automatic stencil support. Additionally, the tooling provides effective support for direct-imaging-type enclosed print heads.
Figure 2. Measurements of solder paste at three points on a board [a) upper left; b) lower center; c) upper right] supported by fully compliant tooling during assembly, find no difference in heights printed.
Initial lab testing of the new tooling system exhibited good results in the control of solder paste height across the board (Figure 2). The microphotos show the board's tooling side, which generally requires full-contact support. Typically, with these board types, it is very difficult to get a low variation in print heights. However, after the board is printed with the support of the new tooling system, the heights are exactly the same when measured in the upper left (Figure 2a), lower center (Figure 2b) and upper right (Figure 2c) points on the board.
Although paste height measurements were not available for this test, end-of-line defect data were collected and compared to those for the same products from the previous three months when manual magnetic tooling was used. After two weeks, the comparative results were as shown in Table 2. With three months' prior data, a more reliable comparison was achieved, displaying improvements from 1.3 to 10 times in the average rate of defects per unit (DPU). An ongoing bridging problem was eliminated, and a fine-pitch QFP (located directly above a like device on the tooling side) that had been producing excessive paste deposits because it was not adequately supported with noncontact-type tooling was resolved.
Force Comparison Lab StudyWhen researching the force limits surface mount devices (SMD) can withstand, a good guideline is the pick-and-place equipment nozzle force (Table 3). To measure the forces applied during a typical print stroke using standard tooling, an array test was devised with two objectives:
- Quantify the force exerted on a pin during printing compared to the pick-and-place equipment nozzle force
- Collect data demonstrating that compliant tooling use would not damage components.
The test method attached sensitive force devices to the pins' tops and monitored the force applied by the squeegee passage. A fixed matrix array featured 3 mm diameter pins spaced 15 mm apart.
As illustrated in Figure 3, the pin setup force measured 0.135 Kgf, or about one-half the lowest nozzle force. Additionally, forces applied during the print stroke did not affect the component directly, but were dissipated through the stencil, board and surrounding pins. It was concluded that minimal forces are applied to the components during the paste-deposition process.
From the test data, a "parallelism" issue arose, i.e., a difference in applied force from the left side of the squeegee to the right side. The highest force was 0.14 Kg, which would reduce if absolute coplanarity was achieved between each pin and would completely eliminate any parallelism issue. Logically, a compliant tooling system would overcome testing discrepancies and exert even lower forces at each pin. This is because a balanced force application provides better pin-to-pin contact.
Figure 4. The percentage of solder volume deviation around the nominal when running with standard tooling and compliant tooling is illustrated. Both a squeegee and an enclosed print head are used.
Test results concluded that, with fixed-tooling support, any variation in the tooling height or the board's support surface will create high pressure points. Tooling that is not machined level (or solder paste and other debris deposited on the top of the support) can cause tooling height variations. Variations in the board's contact surface can result from various factors, including soldermask, part nomenclature, hot-air solder leveling (HASL) finish and labels. Because such variations in the support area create differing pressure points, they have a direct correlation to deposited solder paste height. The effect basically is the same as changing squeegee pressure.
Field Study Confirmation and Conclusion Additional testing of the compliant tooling system vs. standard tooling, using a squeegee and an enclosed print head, was conducted in the field on a product run with an in-line volumetric measurement system (Figure 4). In both laboratory and field testing, fully compliant tooling provided effective support during second-side paste deposition for heavily populated boards. The tested system did not compromise previously placed components and permitted more consistent paste deposition, improving DPU rates. Stencil support to prevent coining was an added benefit. System setup was rapid and required no programming, reducing time and cost over manual, custom and programmable systems. Fully compliant tooling holds significant promise for manufacturers requiring bottomside support for boards with limited real estate.
- DEK 265.
STEVE BECK, a manufacturing engineer, may be contacted at Benchmark Electronics, 4065 Theurer Blvd., Winona, MN 55987; (507) 453-4749; E-mail: Steve. Beck@bench.com. RICHARD LIESKE, applications engineering manager, and RICKY BENNETT, manager of the Advanced Products Div., may be contacted at DEK USA, 8 Bartles Corner Rd., Flemington, NJ 08822; (908) 782-4140; Fax: (908) 782-4774; E-mail: rlieske@dek.com or rickybennett@dek.com.