Filling the Gap: Underfill Rework

Ball grid arrays (BGAs), chip-scale packages (CSPs), flip chips, and other component packages on handheld devices are commonplace today in consumer products, including GPS, cellphones, and computers. In addition, these devices are being used in high-end handheld devices, such as military personnel communicators or space data collection devices. Due to handling and the brittle nature of lead-free solders and packaging materials, these handheld products have used underfill to withstand the mechanical shock or impact when the devices are dropped or struck. The underfilling of these component packages to provide a compliant layer between the package and PCB is one method for increasing the reliability of these interconnections. Figure 1 shows a package before being underfilled, and Figure 2 illustrates the underfill removed.

Fig1-Wettermann-Sep2018.jpgFigure 1: Underfilled device prior to rework.

Underfill is a polymeric material used to fill the gap between the electronic component body and the PCB and encapsulate the solder joints underneath the package. This material enhances the reliability of the component subject to mechanical shocks and impacts by distributing the forces. In addition, it distributes the thermal stresses caused by the coefficient of thermal expansion mismatch between the component and PCB. Typically, underfills have a high glass transition temperature (Tg), a high modulus (E), and a matched coefficient of thermal expansion (CTE) with respect to the solder. Underfill is the “bumper” and distributes the stress more uniformly on the solder joints, thereby increasing the reliability of the solder interconnections.

While there are reworkable underfills in theory, the practical challenges still make reworking an underfilled component very cumbersome and with low resultant yields. The concept of these reworkable underfills is that the material can be thermally decomposed at low temperatures and then removed using various solvents or chemicals without disturbing the solder joints or destroying the substrate or neighboring components. While the marketing material may indicate that the material is reworkable, rework process challenges are numerous.

Fig2-Wettermann-Sep2018.jpgFigure 2: Device location after component removal and site preparation.

Reworking these underfilled devices creates challenges for the rework technician. One of the biggest issues they face is that the softening point of the underfill is below the solder liquidus temperature. In practice, this means that when the device is reworked at one location on a PCB, a neighboring or mirrored underfilled device may reach the softening point and become plasticized. When this happens, the underfills will exhibit lateral pressure underneath the underfilled component. This presents a problem when the solder reflow temperature is reached on a neighboring underfilled device. This force presses on the reflowed solder, which causes the solder to squirt out from the proper location and solder to be pushed to neighboring non-common solder locations. This phenomenon creates a solder short.

A second challenge in reworking an underfilled 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 necessary to remove the underfilled device from the board. The force of the pick-up tube nozzle 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 surface 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 neighboring devices may be destroyed.

A third challenge with reworking underfilled components is that while one method or solvent may work on one type of underfill, this same material set process may not work on another board as they are constantly being changed. New generations of packaging materials and technology require new underfills that work with new alloys and solder masks, finer pitches, and ever thinner substrate materials. This will cause the challenges in reworking underfilled components to be ever present.

Rework technicians must take into account a variety of factors when considering whether or not to rework an underfilled component. However, without a full understanding of the underfill characteristics and a process 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.

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2018

Filling the Gap: Underfill Rework

09-21-2018

Rework technicians must take into account a variety of factors when considering whether or not to rework underfilled components, such as BGAs, CSPs, flip chips, and other component packages on handheld devices. But without a full understanding of the underfill characteristics, expect the outcome to be low yields unless the board was designed with reworkability in mind.

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Replating of Gold Fingers: Getting the Shine Back

07-30-2018

There are several instances where the gold contacts on PCBs need to be replated, and IPC A-610 discusses several of these cases. This column by Bob Wettermann discusses gold replating of defective contacts caused by processes such as wave or selective soldering, or plating.

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Masking of Conformal Coating During Assembly and Rework

06-11-2018

Masking of printed circuit boards post rework/repair as well as for initial PCB assembly is often required if the PCB is to be conformal coated. If a board that has conformal coating on it needs to be reworked or repaired, the conformal coating needs to first be removed before the operation of rework or repair can take place. This article centers around the various options for conformal coating masking via a liquid application process.

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Device 'Dead Bugging'

04-20-2018

"Dead bug" attachment of electronic components is a way of building functioning electronic circuits by soldering the parts directly together or by soldering miniature jumper wires between the component leads and the PCB lands instead of the traditional surface mount or through-hole soldering of components onto a PCB.

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PCB Pad Repair Techniques

01-08-2018

There are a variety of reasons behind pads getting "lifted" completely or partially from the laminate of a PCB. Per the just revised IPC-A-610 Revision G, a defect for all three classes occurs when the land is lifted up one or more pad thicknesses. Lifted pads can occur when a device has been improperly removed or there is a manufacturing defect in the board construction. In any case, as with any repair, the ultimate decision on the ability to repair the pad lies with the customer.

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