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Your local PCBA needs your support; proper support can help prevent defects and eliminate setup errors.

Does your surface-mount assembly process have bouncers? I'm not talking about the big guy who ejects disgruntled customers from the shop floor; I'm talking about boards that are not properly supported during the printing and pick-and-place processes. The lack of proper support and resulting seismic activity of the printed circuit board assembly (PCBA) contributes to process defects and impacts yields.

Why is Proper Support Necessary?

In the printing process, proper PCB support is necessary for accurate solder paste transfer. We have highly accurate machines and accurately tooled stencils, but, if the PCB is not supported to minimize deviation from flatness, much of that inherent accuracy is lost. Paste transfer and the thickness and definition of the print will be adversely affected. At the least, expect to see insufficients; if the situation is really bad, print alignment might be off across the entire PCB.

Sufficient PCBA support is even more necessary when doing a double-sided board. Regardless of how flat the PCB originally was from the fabricator during first-side assembly, the PCBA has passed through reflow and resided above the glass transition temperature (Tg) of the laminate. For FR-4, this temperature is usually 150-170[degrees]C and is the point where the laminate loses its rigidity. Although it regains its rigidness when the temperature descends below this range, a greater degree of warpage has been introduced. When doing the bottom side, more deviation from flatness and a greater need for good board support exist.

Component placement also requires good PCBA support. The dynamics of the z-axis down-force can be enough to cause movement of previously placed parts as well as misplacement of the part being placed. Nothing hurts like spending big bucks on a pick-and-place machine that can place within 4 microns only to have the part bounce off location.

[ILLUSTRATION OMITTED]

One company, CyberOptics, has developed an embedded process verification system (EPV) for component placement systems that is comprised of an on-board sensor system monitoring the point of placement on chip-shooters and flex placement systems. The system can sense and measure the magnitude of board vibration during the placement operation. On a poorly supported PCBA, vibration can be in excess of [+ or -]0.4 mm.

Such excess vibration is often the fault of setup, a human factor in automation. If the technician fails to put in the proper support pins or blocks for an application during setup, board vibration will likely occur.

Available Support Options

A number of approaches can alleviate this problem on both printing and pick-and-place systems, as well as on adhesive dispensing systems or other process equipment requiring good support of the PCB substrate. The most economical approach is to place application-specific Mylar[TM] overlays on the tooling plate and have cutouts for the support pins/blocks. Color-coding the perimeter of the cutout prompts the tech to choose the correct support piece.

One high-end option available on some printers is an on-board pick-and-place subsystem to automatically place support pins during setup. Once the system is programmed correctly, it works well and with repeatability.

Another on-board approach uses pneumatically actuated pins in programmed locations that come up during set-up. However, in addition to being expensive, neither system is easily field-retrofittable.

[FIGURE 1 OMITTED]

An interesting solution is a modular, retrofittable, inexpensive system that uses a modular block with mechanically actuated, spring-loaded support pins (Figure 1). The user puts in the blocks to cover the PCB area. The modules mount to the machine plate either via tooling or magnetically. With a known good board in place, a cam is rotated and the pins seek the proper height.

[FIGURE 2 OMITTED]

If, for example, the application is the second side of a double-sided board, the pins will seek the height of the substrate as well as components and memorize the proper z-position (Figure 2). The pins are non-abrasive, so no damage is caused by contact with the board or components. Some of these systems are also available with side-snuggers.

Automatically actuated versions of this method cost a bit more, but are field-retrofittable by the user. Either way, the technology is an economical, efficient means of accomplishing good support for printing, adhesive dispensing and pick-and-place applications. Besides preventing defects, the systems expedite setup and make the operation about as idiot-proof as possible.

So, give your PCBs the support they need. There are plenty of board support suppliers out there, including Production Solutions, Grid-Lok, DEK and Transition Automation, among others, so you have no excuse. It's better than being caught between a rock and a hard place. Remember, we're all in this together.

Phil Zarrow digs cool gadgets that actually work. He is president and surface-mount process consultant for ITM Inc., Durham, NH; (603) 868-1754; email: phil_zarrow@ITMconsulting.org.
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Title Annotation:On the Forefront
Author:Zarrow, Phil
Publication:Circuits Assembly
Date:Dec 1, 2003
Words:788
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