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74 SMT007 MAGAZINE I SEPTEMBER 2018 low temperatures and then removed using var- ious solvents or chemicals without disturbing the solder joints or destroying the substrate or neighboring components. While the mar- keting material may indicate that the materi- al is reworkable, rework process challenges are numerous. Reworking these underfilled devices creates challenges for the rework technician. One of the biggest issues they face is that the soft - ening point of the underfill is below the sol- der liquidus temperature. In practice, this means that when the device is reworked at one location on a PCB, a neighboring or mir - rored underfilled device may reach the soft- ening point and become plasticized. When this happens, the underfills will exhibit later- al pressure underneath the underfilled compo- nent. This presents a problem when the solder r eflow temperature is reached on a neighbor- ing underfilled device. This force presses on the reflowed solder, which causes the solder to squirt out from the proper location and sol - der to be pushed to neighboring non-common solder locations. This phenomenon creates a solder short. A second challenge in reworking an under- filled component is the tack and adhesive strength of the bond created between the underfilled board and component. This force causes several practical process challenges in the rework process. The first is the force nec- essary to remove the underfilled device from the board. The force of the pick-up tube noz- zle on the rework system will not be enough to pick the reflowed component from the board because the underfill holds it tightly to the board. This means that a twisting and prying force must be applied after the solder liquidus temperature is reached. These forces are such that neighboring devices can be destroyed, the mask can come off of the component or the board, or neighboring devices may even be disturbed in the rework process. In addition, when the device is finally removed from the board, both the board sur- face and the bottom of the device may need to have the underfill scraped off its surface. This is done with lateral force and heat, both of which may scratch the mask or lift pads. When cleaning the device location on the PCB of the remnant underfill, solder mask may be also be scratched, pads may be lifted, or neigh- boring devices may be destroyed. A third challenge with reworking under- filled components is that while one method or solvent may work on one type of under- fill, this same material set process may not work on another board as they are constant- ly being changed. New generations of pack- aging materials and technology require new underfills that work with new alloys and sol- der masks, finer pitches, and ever thinner substrate materials. This will cause the chal- lenges in reworking underfilled components to be ever present. Rework technicians must take into account a variety of factors when considering wheth- er or not to rework an underfilled compo- nent. However, without a full understanding of the underfill char acteristics and a pro- cess development distinctive to the specific board material, underfill, and board design, expect the outcome to be low yields unless the board was designed with reworkability in mind. SMT007 Bob Wettermann is the principal of BEST Inc., a contract rework and repair facility in Chicago. To read past columns or contact Wettermann, click here. While the marketing material may indicate that the material is reworkable, rework process challenges are numerous.

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