How Much is Too Much?

One of the typical questions process engineers pose regarding the PCB rework process is, "How many heat cycles are too much?” Asked in another way, the question is, “How would one define a limit on the number of times a PCB can be reworked while still being reasonably assured that the reliability has not been impacted based on its operational environment?"

The answer will depend on a multitude of factors, but it is largely influenced by the board’s function in the end-use operating environment. For example, an airplane control module that is protecting passengers while in flight (Class 3) has a different set of maximum conditions compared to a giveaway toy that came from the local fast food eatery (Class 1). In reviewing this topic, consider that rework cycles consume part of the useful thermomechanical life of the PCB.

There are several factors to consider when calculating the maximum number of allowable rework cycles. Some of the most relevant factors include PCB design, materials (including the components involved in the design of the PCB), and the number of heat cycles or heating and cooling cycles the board has experienced. The multitude of factors and there being no one number is also supported in the rework and repair guidelines document, IPC-7711/21, which states as follows, "This document does not limit the maximum number of rework, modification, or repair actions to a printed circuit assembly."

Wettermann_Fig1_Jan2019.jpg

Figure 1: How many times should I attempt to rework a PCB?

PCB design has an impact on how the many cycles should be allowed for rework. There are hearty laminates that can take multiple heat cycles. Higher layer count PCBs with high aspect ratios through vias are more prone to failure after only a few soldering cycles if the materials of construction are not robust. In general, larger pads hold up under more heat cycles; however, with smaller pads, there is a greater chance of board delamination after only a few cycles. Testing and experimentation via life cycle testing will show what is right for your specific application and end-use operating environment. A well-chosen material set will, in most cases, withstand up to six heat cycles (three rework cycles) for the majority of Class-2 or Class-3 designs.

The materials included in the design of the PCB (including the components and boards) have an impact on the maximum number of rework or heat cycles that an electronics assembly can withstand. As a reminder, an entire assembly can be compromised if only a single component is damaged. Make sure to look up the specifications for all of the components to investigate the temperature limit specified by the component manufacturer. It’s important to note here that passive components may have a lower time-over-temperature exposure limit than larger active components. This is known in the component specification world as the process sensitivity level (PSL). Furthermore, check what these values mean for other elements on a PCB, which are further defined in the IPC J-STD-075: Classification of Non-IC Electronic Components for Assembly Processes. During a reflow process, the board is stressed as well as the solder joints and all of the components.

The total number of heat and cooling cycles should be part of the decision to limit the number of rework cycles. In general, the typical number of rework attempts (multiply by two to get to the number of heat cycles) for Class-2 and Class-3 PCBs is three. Make sure to enumerate all of the heat cycles the board has already gone through. For a typical double-sided PCB, this could include both the primary side component placement and reflow as well as the secondary side placement and reflow.

Develop a profile for each of the processes using a thermal profiler to prove that you have not exceeded any of the component temperature/time limits. If we simply remove and replace one of the devices, then the count of reflow cycles already numbers four. For BGAs and other active devices, the usual number of heat excursions is equal to the number of times the die is exposed to the liquidus temperature of the soldering alloy used. However, wave soldering, baking, and conformal coating curing processes may heat and relax the PCB. If reballing of the device is required, then another two heat cycles—one for the removal of the BGA balls and the other for the re-attachment reflow—must be added to the total. There are some ball removal processes that do not raise the die temperature above liquidus.

In addition to the technical limitations placed on the maximum number of rework cycles, there are also economic decisions that may drive the cost versus benefit of a rework process. Many times, the alternatives evaluated include the removal and replacement of the device or scrapping the board and replacing with a new assembly. At times, the cost and availability of these options push the decision in a certain direction. PCB rework yields, overhead costs, and opportunity costs in using labor (which could be producing higher margin PCBs) enter the decision-making process. Thus, the question that needs to be answered is, "When is too much too much?"

Further Reading

1. IPC—Association Connecting Electronics Industries. “IPC-7711/21: Rework, Modification, and Repair of Electronic Assemblies.” November 1, 2011.

2. Coyle, R., Meilunas, M., Popowich, R., Anselm, M., Read, P., Oswald, M., & Fleming, D. “Interconnection Reliability of Interposer and Reballing Options for BGA Backward Compatibility.” SMTA International proceedings, October 14, 2012.

3. Ma, H., Xie, W., Subbarayan, G., & Lieu, K.C. “Effects of Multiple Rework on the Reliability of Lead-free Ball Grid Array Assemblies.” IEEE 61st Electronic Components and Technology Conference, 2011.

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2019

How Much is Too Much?

01-09-2019

One of the typical questions process engineers pose regarding the PCB rework process is, "How many heat cycles are too much?" Asked in another way, the question is, "How would one define a limit on the number of times a PCB can be reworked while still being reasonably assured that the reliability has not been impacted based on its operational environment?" Find out how.

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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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