Proper Thermal Shielding Yields Highest Rework Results

There are numerous "gotchas" if the rework technician does not care for components and materials neighboring the component rework area. If this care is not taken, the rework component might have been properly reworked, but the PWB itself may not be functional due to thermal damage. While PCB rework processes using best-in-class methods, materials, and equipment may have a high yield, an even higher yield can be achieved by protecting surrounding components.

Fig1-Wettermann-21Nov2018.jpgFigure 1: Skewed component due to proximity to the one being reworked and improper shielding.

During component rework, taking extreme care of nearby components—whether they are adjacent to the device being reworked or on the opposite side of the board—is required to avoid collateral heat damage, inadvertent reflow, and altered characteristics, such as discoloration or part skewing. Damage can be done to the part physically when the heat from the rework source is beyond the range of what the component is rated. This is common with connectors, relays, and batteries. In addition to component damage, de-wetting, pad damage, starved solder fillets, and component surface finish oxidation can result.

In some cases, the damage is unseen in the form of increased intermetallic growth in the solder joint, which may impact the reliability of the assembly. In end-use environments where reliability is critical, this increased reliability risk must be investigated for its impact. Also, a process capability study might have to be completed to verify the rework process.

Further, there can be other materials damaged on the assembly that are part of the proper operation and design for its end-use operating environment. These materials can include, but are not be limited to, the following items:

• Underfill materials underneath the components

• Staking materials on the PCB

• Insulative coating materials

• Conformal coatings

• Thermal grease

These materials can dry up, become nonfunctional, soften, or migrate to other areas of the assembly, which all have a detrimental effect. For example, underfill materials have a softening temperature below that of the reflow temperature of the solder. When an underfilled device reaches this temperature, it pushes out solder that reaches reflow temperature and causes shorts underneath the neighboring device. Thus, it is important to know the materials on the board and be aware of technical information on the datasheet from the material supplier or have experience about the materials’ properties.

There are numerous steps the rework technician can take to avoid these neighboring reflow "gotchas." One of the first steps to prevent having to scrap a board or perform rework a second time (if allowed by the customer) is to investigate the materials and components on the board before starting the rework operation. It is better to take a moment to question the material composition of nearby components and pull a few component or material datasheets then to start again.

Fig2-Wettermann-21Nov2018.jpgFigure 2: Use shielding materials, like gel, to mitigate the effects of heat in the rework area.

After the risk areas are identified, protect at-risk parts by either removing or shielding them. The use of ceramic non-woven tape, a metal shield, or shield gel has been shown to be the most effective thermal shields [1]. Finally, placement of thermocouples around the component being reworked when performing rework profiling will help identify issues. If a hot-air or an infra-red (IR) reflow source is used, this will help to pinpoint potential problem areas instead of guessing.

The reflow of surrounding devices in the rework area is a yield detractor in PCB rework. If you are not careful with the reflow source being used, nearby materials may be destroyed or cause the PWB not to meet the initial specifications or design of the assembly. However, careful planning, shielding, and sometimes removing neighboring device or material will ensure the highest possible rework yield.

References

1. Shielding Effectiveness of Polyimide Tape During Rework by Gaynor Adam and Robert Wettermann, Circuits Assembly, October 2014.

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2018

Proper Thermal Shielding Yields Highest Rework Results

11-21-2018

There are numerous "gotchas" if the rework technician does not care for components and materials neighboring the component rework area. However, careful planning, shielding, and sometimes removing a neighboring device or material will ensure the highest possible rework yield.

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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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