Knocking Down the Bone Pile: Methods for Underfilled Component Rework

Products such as engine control modules, drones, smartphones, and other handheld communication devices, which are designed for high reliability and require high processing power, often have a BGA or CSP package as the processor. Underfill has been a solution at the package level protecting these devices from the coefficient of thermal expansion (CTE) mismatch between the device and PCB or between the die and the component substrate for flip chip packages. Stress caused by CTE mismatch redistributes the stress from the bottom of the solder spheres to the entire component. Underfill also provides for a mechanical bond between these surfaces while providing very good protection for the solder joints against both mechanical and thermal strains as the product experiences vibration, drops, or thermal shocks.

Underfill is typically constructed from either an epoxy-based or other polymeric-based material and is dispensed in a controlled fashion underneath the component. Through capillary action and under increased temperature, the underfill makes its way underneath the component. It is typically cured by either thermal or thermo-sonic energy.

The downside of using underfill is that it causes the rework process to be extremely difficult. While some underfills are categorized as “reworkable” this does not mean that the underfill removal process is not without extreme challenges. There are several problems which arise from removing an underfilled component regardless of whether it is “reworkable” or “non-reworkable.” The problems are fairly numerous when the underfill is heated and moves around, breaking the bond between board and component. Some of the most common problems include:

  • Twisting motion to break board/device bond during the component removal process may damage board or neighboring devices
  • Using a sharp tool to “cut” the underfill so the component can be removed may damage neighboring components or the PCB
  • Mechanically scraping or cutting away the underfill may cause damage to PCB or neighboring components
  • The mechanical bond of the underfill to the PCB may cause the lifting of pads (Figure 1), laminate damage, or solder mask damage (Figure 1)
  • The time-consuming nature of removing the underfill makes the rework process beyond economic repair
  • Heating the solder underneath the BGA/CSP/QFN to a liquidus temperature for removal causes the underfill to begin to reflow, “pushing” solder out from the designated pad areas
  • Neighboring underfilled components can be pushed off their pads during the removal process

Wettermann_Nov_Fig1_cap.jpgThere are numerous methods which can be used to rework these underfilled devices. Each method has both advantages as well as drawbacks.

The method used most often for reworking underfilled devices involves the use of a heat source to reflow the BGA while simultaneously creating a parting line in the underfill between the component and the PCB. This will release the component from the pads on the PCB, as the softened material still adheres to both the component and the board. Either the BGA rework system has a twisting motion to get the component removed or a bimetallic reflow nozzle “grabs” the suspect component for removal. After component removal, the site is dressed using a hot air source along with a tool such as a solder extractor or a hobby knife, or by utilizing a specific soldering iron chisel tip along with solder braid. If no BGA rework station is available, then a non-controlled heat source such as a heat gun can be used to remove the component from the board followed by clean up. These processes require care and skill by the operator to not damage the board or neighboring components.

Another method which can be used to remove the BGA by breaking the adhesive force of the underfill is via a softening agent. A solvent-based chemical—formulated by the underfill vendor—comes into contact with the underfill and softens it. This allows for easier removal of the underfill, thereby reducing the likelihood of component, mask, or pad damage. One of the drawbacks to this methodology is being able to get the softening agent to all areas of the underfill, as the hardened underfill blocks the liquid from getting to all areas. Furthermore, depending on how aggressive it is, the liquid can damage components and device markings as well as other coatings. Post softening agent application cleaning is also a challenge as analytical testing needs to be part of the process qualification. In many cases, the reliability risks imposed by this method are not worth the process time-saving rewards.

A further removal method of the component relies on laser ablation. In this method a directed laser source is programmed to scan the surface area of the laser back and forth to disintegrate or ablate the component, underfill, and some cases, the solder balls. At low laser flux (low laser energy density, w/cm2), the material is heated by the absorbed laser energy and sublimates. The laser beam is pulsed, electronically controlled, and the beam steered to not destroy neighboring components. This method is useful in applications where either the vibration of the board in precision milling presents too high a reliability risk or when either the component or board warpage is extreme.

Wettermann_Nov_Fig2_cap.jpgA more recent addition to the underfilled BGA/CSP removal process list of alternatives is the use of a precision mill to cut away the device. Typically, “non-reworkable” underfill along with the component and solder is milled off the board. The board is fixtured properly to retain a flat surface, typically with the use of a vacuum fixture. Height sensors make sure the mill does not drive too far down into the board and damage it. Vacuum is also supplied near the cutting area to keep debris off the PCB. As reflow temperatures are not reached, lesser board damage results. The resulting vibration of the milling process and its effect on the board need to be investigated for the end use operating environment.

Wettermann_Nov_Fig3_cap.jpgThere are numerous methods for reworking underfilled area array and bottom terminated components. A skilled, experienced rework technician with several methods, including the machinery, available to them will determine the highest yielding method in the fastest amount of time.

This column originally appeared in the November 2021 issue of SMT007 Magazine.

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2021

Knocking Down the Bone Pile: Methods for Underfilled Component Rework

11-17-2021

Products such as engine control modules, drones, smartphones, and other handheld communication devices, which are designed for high reliability and require high processing power, often have a BGA or CSP package as the processor. Underfill has been a solution at the package level protecting these devices from the coefficient of thermal expansion (CTE) mismatch between the device and PCB or between the die and the component substrate for flip chip packages. Stress caused by CTE mismatch redistributes the stress from the bottom of the solder spheres to the entire component.

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Knocking Down the Bone Pile: X-ray Imaging and BGA Rework

09-08-2021

X-ray imaging prior to the removal of a BGA for rework will help the rework technician point out potential issues which may be challenges to successfully removing and replacing the BGA.

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Knocking Down the Bone Pile: Cleaning of ‘No Clean’ Fluxes in PCB Rework

07-26-2021

The original intention of a “no clean” solder was to eliminate the post PCB assembly cleaning process while still not risking any performance or long-term reliability degradation. Some industry surveys indicate that about one-half of assemblers using no clean flux chemistries clean the PCB after assembly.

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Knocking Down the Bone Pile: Solder Excavation and Rework

05-10-2021

In order to properly perform rework—the removal and replacement of a component on a PCB—the remnant solder needs to be properly removed after the component has been desoldered and removed. Bob Wettermann breaks down the methods.

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Knocking Down the Bone Pile: Salvaging Components for Other Uses

03-04-2021

Electronic components and their availability (or rather their lack of) have been in the news recently. Automotive suppliers are struggling with their supply chain as electric vehicle production, and the associated consumption of electronic components continues to expand.

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Knocking Down the Bonepile: Fixing Vertical Hole Fill in Plated Holes

01-10-2021

For PCBs with larger thermal mass—such as found in high layer count boards or boards with higher weight copper layers—proper and consistent hole fill can be a challenge. It is critical to make sure that these non-visible defects do not become quality escapes while also making sure the proper rework techniques are applied; to get these plated holes properly filled is important.

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2020

Knocking Down the Bone Pile: 5 Habits to Make Your Soldering Iron Tips Last Longer

11-02-2020

Poorly maintained soldering iron tips have real costs associated with their lack of care. To maintain the integrity of the soldering joints and prevent the tips from becoming a runaway consumable expense, Bob Wettermann shares several areas of tip care that can prolong their life.

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Knocking Down the Bone Pile: PCB Rework of 0201 Packages

09-07-2020

As electronic passive components continue to shrink in size, methods for their rework need to be developed by electronic manufacturers to maintain and support PCB assembly processes. Bob Wettermann compares and outlines a few of these rework methods.

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Knocking Down the Bone Pile: Removing Conformal Coatings for PCB Rework

07-27-2020

When the removal and replacement of components due to field failures or manufacturing defects needs to occur, the overlaying conformal coating layer first must be removed before being able to remove and replace a component. Bob Wettermann explains.

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Knocking Down the Bone Pile: Getting to the Root Cause of BGA Assembly Problems

05-04-2020

When potential process defects begin showing up underneath BGAs in electronic assemblies, there are numerous failure analysis tests that can be used to troubleshoot process problems. These investigative methods begin with non-destructive test methods and progress to destructive methods as some of the possible root causes are eliminated.

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Knocking Down the Bone Pile: Underfill Rework and Solder ‘Squirt Out’

01-02-2020

One of the toughest rework challenges is removing and replacing components on PCBs with underfilled components. Many times, underfill is used to provide a shock barrier to component solder joints of handheld electronics, such as notebooks, tablets, and phones. This underfill is added post-test in the assembly process and is dispensed underneath components, such as BGAs, QFNs, and LGAs.

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2019

Knocking Down the Bone Pile: Electronics Assembly Industry Outlook

12-17-2019

Geographically, our products go directly into the market around the world, our rework and repair services are a harbinger of the EMS build market, and our training services are hyper-focused in the Midwest of the United States. Therefore, we see much of the activity in the global electronics supply chain. There are numerous PCB rework/repair challenges being faced by North American customers. One trend has to do with increasing package sizes, which are being driven by the market desires. In the past five years alone, the state-of-the-art semiconductor package has gone from approximately 10 to 30 billion transistors on a single package.

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Knocking Down the Bone Pile: Process Methods for Reworking High Lead Count SMT Parts

10-09-2019

There are numerous methods for getting the solder onto the right pads in the right volume during SMT rework of high pin count or very small footprint SMT devices. The most common types of solder deposition include printing, dispensing, and hand soldering. Each of these methods has pros and cons, depending on a variety of factors in the rework process.

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Knocking Down the Bone Pile: BGA and PCB Warpage—What to Do

07-10-2019

Warpage of BGA packages or PCBs can occur when any heating and subsequent cooling cycle is gone through. This may leave the package to bow in the middle. Pushing the corners up or downward will show up in bridging (caught on X-ray) or cause opens that would show up on endoscopic or visual inspection. Here's what you need to do.

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Knocking Down the Bone Pile: Straightening Out Those Corners

05-22-2019

A PCB can be dropped, dinged, or mishandled as it is placed into a board carrier in the PCB assembly operations area. When the laminated material is damaged in this manner, can it be repaired? The answer, like most engineering answers, is that it depends. Read on.

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Inspection of BGAs After Rework

03-21-2019

After removing and replacing a ball grid array (BGA), the acceptability of the interconnection of the solder balls to the PCB should be assured, because this assurance and the criteria for that assurance are the customer’s outgoing inspection criteria.

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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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