Keeping Things on the Level

Written by Rush PCB on . Posted in PCB Assembly and component

Sometimes, in PCB assembly, it’s not the layout of the SMT PCB that creates issues, but the design of the part itself, or the plan for the part’s location, given its dimensions. We have to ask ourselves, sometimes, “What were they thinking?”
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In this case, a customer’s BOM called for a part (an RF200 module with through-hole pins) to be mounted onto a PCB. At one end is a bulky SMA connector that due to its size exceeds the thickness of the module. The SMA connector faces inward on the PCB; it’s not mounted to hang over the edge. As a result, the SMA connector bottom side touches the board and props one end of it up; it doesn’t allow the module’s pins to be properly inserted into their corresponding PTH barrels on the board. One end of the SMA is pointing upward on an angle like a missile-launcher.

This is obviously not acceptable in circuit board assembly; not only is customer access to the connector compromised, but the module cannot be mounted in a planar fashion and having some of the pins fully inserted and some halfway out of the barrels, with one end of the module elevated, is certainly not acceptable.

Here at RUSH PCB Inc., the fix was relatively easy; we recommended that the customer allow us to use two (2) single-row socket pin adaptors to provide the standoff necessary to keep the SMA connector from touching the board while at the same time allowing easy and unobstructed access to the connector.

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Two socket headers were used, corresponding to the module’s two rows of pins. Not only did this provide the needed standoff, without creating any other issues, but it also allowed the customer the potential for removing or replacing the module in the socket pin adaptor in the future without serious rework issues, since it’s a mechanical mounting. It’s also a robust electronic assembly connection in terms of strength and durability, and the module is completely planar with the surrounding PCB surface.

- Roy Akber, CEO, RUSH PCB Inc.

Stop the Drop: Fixturing a Heavy SMT Component Saves Time, Money

Written by Rush PCB on . Posted in PCB Manufacturing

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Double-sided SMT PCBs are usually no problem to solder; two passes, and the bottom-side components are held in place with SMA adhesive. But occasionally, a large or heavy component on the bottom side, such as an inductor, poses a problem because it will drop off during top-side reflow. In one particular case, an inductor with an open bottom (with inner coils visible) couldn’t be fixed in place with adhesive. Its two pads didn’t have enough solder surface tension when liquidus to hold it to the bottom side of the board during the board’s second run in the reflow oven with part turned upside-down. What to do?

The solution was a universal reflow fixture with support bars, located beneath the part to support its weight during reflow so that it would not pull loose and drop. The fixture had to hold it firmly in place and in location during the second pass through the oven.

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Other solutions were either unsuitable or added an extra process step. The use of adhesive, as mentioned above, was not practical, since there was little available component body to attach to, and one did not want any foreign material, i.e. adhesive, to get inside the component body with the exposed coils. Nor was the use of a higher-temperature solder practical; the component is a standard part and simply needed to install with the other components without additional steps or complications.

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Adapting the universal fixture to hold the inductor in place was an easy matter. The flexibility of universal fixtures is such that they can be adapted to support a wide variety of component sizes and shapes effectively across a wide range of board sizes, layouts, etc. They are also much less expensive than a custom fixture, and readily available for use. Consider this in contrast to an expensive custom fixture that will require on average 2 – 5 days to fabricate.

The universal fixturing will do the job, and be re-usable; it’s a one-time investment that can be used over and over again for a multitude of different assemblies. Saving production time and tooling costs has a positive effect on profitability, and at the same time, the highest level of product quality and reliability is maintained. Obviously, not all assemblies require fixturing, only those with special requirements like this one that cannot be processed as simply as most SMT assemblies can.

Roy Akber
CEO
RUSH PCB Inc

Solder Mask Design Changes Help Prevent BGA Pad Lifting

Written by Rush PCB on . Posted in PCB Design

Damaged and/or lifted BGA pads on a PCB Assembly are a common problem during BGA rework. Lifted pads are most commonly the result of overheating during the rework process, and also occasionally due to poor adhesion of the pad to the base material of the PCB. Saving a PCB with lifted pads goes beyond rework and into the realm of circuit repair, a more costly, time-consuming, and complex process.

One proven solution to this problem, developed here at RUSH PCB Inc., is to modify the solder mask pattern to allow a slight overlap of the mask over the edges of the BGA pads. This slight encroachment or ‘lip’ serves as an anchor for the BGA pad, keeping it down on the PCB so that it doesn’t lift off even if its adhesion to the PCB has been compromised by overheating. This mask encroachment also provides protection for traces between the pads in addition to securing the land to the PCB. But for this approach to be effective, the size or surface area of the BGA pads must also be expanded slightly to accommodate the overlap, which should be approximately 2 mils overall.

BGA pad size can be increased by changing the PCB design to a via-in-pad approach. Via in pad makes additional PCB real estate available to increase BGA pad size on the circuit assembly. In addition to creating pad area for mask overlap, the larger pad surface area helps ensure better adhesion of the pad to the PCB assembly. Even though this change could add slightly to the cost, that additional amount would be offset by lower rework volumes, especially since rework is expensive and negatively impacts production flow.

This approach is also most effective if a smaller BGA ball size is used. This makes the maximum amount of pad area available for solder mask overlap, while at the same time leaving enough solderable pad area to form robust solder joints on the electronic assembly. In most cases, the smaller BGA sphere size can be accommodated. When this is not possible due to component-induced restrictions, then the need for additional pad size can be addressed during the initial PCB design stage.

From a PCB design perspective, the mask design modification to create the 2-mil overlap is achieved at the solder mask Gerber design stage and is created during the mask process, and is not affected by the mask application method.

- Roy Akber, CEO, RUSH PCB Inc.