February, 2016
www.us-tech.com
Page 55 The Evolution of Cleaning Methods in PCB Rework By Robert Roush and Paul Wood, Metcal T
he rework process has long consisted of three steps: removal of the inoperative component, cleaning of residual solder from the ball grid
assembly (BGA) pads which held the component in place, and replacement with a new component to create a viable, working PCB. The first and third steps have been automated, and are completed on rework machines with operator assistance. Cleaning, the second step, and arguably the most sensitive and vital when it comes to reclaiming a damaged board, is often done manually by a skilled technician. Cleaning is performed with hand tools, usually a wicking braid and soldering iron, using standard or specialized tips. Obviously, the effectiveness of the operation is highly depend- ent on the skill of the operator, which makes board damage a frequent occurrence. One common scenario in manual cleaning is
the destruction of a pad due to sticking, when the heat of the soldering iron momentar- ily dissipates through the board, causing the pad to stick to the wick and be chipped or to lift off the board entirely when the operator pulls his tool back. This is a highly undesir- able result since destruction of a pad results in a wasted PCB that has just become scrap. In the drive for greater function-
ality, manufacturers are creating thicker PCBs with multiple inner lay- ers of copper. These, unfortunately, also cause heat to dissipate more quick- ly when using hand-held cleaning tools, ensuring that sticking challenges will become even more of a problem. Other problems may include
molten solder flowing into electrical connections and into vias which caus- es shorts in the board. Further, sol- der may be removed inconsistently among the pads which results in poor adhesion of the new components, or portions of the solder resist can be inadvertently removed by the wick which causes solder to flow into elec- trical connections when the board is put back into the rework machine, leading to bridges and shorts. Perhaps an even more insidious
issue is that while many manual cleaning problems can be caught in the inspection process, and the board, which is now expensive scrap is kept from getting to market, another com- mon source of damage is not dis- cernible by visual — or even X-ray — inspection. Pad craters can be created in the fiberglass substrate under the pad due to the stresses caused by the mechanical pressure of operators pressing a little too hard with the sol- dering iron, or by excessively heated tips held slightly too long against the board. With this kind of damage, the pad and the solder balls are still con- nected to each other, but the pad is not fully anchored to the circuit board, leaving it vulnerable to the smallest jolt by the consumer — and the end product brand vulnerable to consumer dissatisfaction. The smaller the pads, the more
susceptible they are to hidden crack- ing, which means that the situation has been and will become more com- mon as pad size continues to decrease.
Manual Cleaning Gets Tougher Over the years, pad size and
pitch size have been reduced signifi- cantly. Many devices now use pads as small as 0.25mm (0.0098-in.) , spaced at intervals of 0.5mm (0.02-in.). New chips are scheduled to be released in the next two or three years that call for pad sizes as small as 0.1mm (0.004-in.) with a 0.3mm (0.012-in.)
pitch. Avoiding hitting the parts with the wick during manual cleaning will likely become impos- sible for even the most skilled technician. Another challenge for traditional cleaning
methods is presented by PCBs with advanced chip technologies. For example, one new technology is providing outstanding improvements in security, making it nearly impossible for criminals to access data stored on the processor. However, due to fac- tors such as differing pad sizes and uneven solder volumes, manual cleaning can be a challenge. Ceramic ball grid array, a specialty process
Open via and solder resist damage.
used in aerospace, military and other high-reliabili- ty applications, is another example. These boards, which use individual components costing tens of thousands of dollars and themselves, with PCBs that can cost half a million dollars or more, are noto- riously hard to successfully clean manually and
Continued on next page
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CARE knows why it’s important that it does.
Your CLEANING problem is our cleaning problem. And we don’t quit or go
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