Knocking Down the Bone Pile: Solder Excavation and Rework

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. This step is important for the following reasons:

  1. The component, especially if it is a small outline package or an ultra-fine pitched component, needs to lie co-planar to the surface of the pad in order to get the replacement component properly aligned.
  2. The remnant solder may already have a thick intermetallic layer which may cause the solder joint to prematurely fail.

There are several methods in use for the removal of the remnant solder including but not limited to vacuum extraction, solder braid (wick) and coupon use. Each method has its own advantages as well as drawbacks.

The solder braid approach is outlined in the IPC 7711 Methods for PCB Rework, section 4.1.3 (Figure 1)—surface solder removal-braid method:

  • In this method, solder braid, which is a copper mesh, is placed onto the fluxed PCB pad. Some braids are pre-fluxed which increases the wicking action.
  • The braid size chosen should be slightly smaller than the pad dimension. It is recommended that paste flux be used to make sure the flux is active during the solder removal process.
  • Make sure the tip temperature corresponds to the reflow temperature of the solder alloy being removed. Move the soldering iron tip up and down perpendicular to the pad with the soldering braid in between the tip and pad.
  • When solder wicks up into the braid, remove the braid from the pad advancing it such that a non-solder filled section of the braid can be used on the rest of the pad.
  • Do not “swab the deck” on multi-leaded components or area array sites moving across the pads as the pads could be lifted or the mask damaged. This operation is operator dependent and requires skills practice in order to not damage the PCB.
  • The solder wick method requires the least amount of equipment investment, but it does require intermediate soldering skills.

Wettermann_Fig1_cap.jpgIn addition to the solder wicking method, vacuum extraction is another means for removing the solder from a pad location. This vacuum can be created by a manual spring-loaded pump, a hand tool, as part of a BGA/leadless device rework system, or as a stand-alone programmable tool (Figure 2).

Wettermann_Fig2_cap.jpgIt is not recommended to use a manual pump as there is a lack of continuous suction at the tip as the solder joint is cycled through several heating and cooling cycles. In the case of the other powered vacuum desoldering tools, a hole in the tip center is used as a vacuum to remove solder which has gone into reflow. Matching the diameter of the tip to the width of the pad is recommended as larger-than-pad-sized tips may burn the PCB laminate. After applying flux to the location, place the heated tip onto the pad gently until you sense the solder is going into reflow. Do not exert pressure onto the pad as it may cause pad damage.

Integrated programmable soldering excavation tools usually have a sensor which keeps the distance of the PCB at a fixed height. Reservoirs which contain the excavated solder need to be cleaned out whether it is a manual hand piece or part of an automated system. The vacuum solder extraction method, regardless of the type of system used, is fast and requires less skill and experience compared to the solder braid approach.

The technique which is used the least for solder removal in PCB rework is the copper coupon method. This method uses a flux-dipped copper coupon to remove all the solder at one time. The coupon is placed onto a BGA rework system nozzle and heated, then slowly lowered onto the surface of the PCB. As it gets in the vicinity of the BGA or CSP pads, solder is wicked onto the coupon. One of the advantages of this approach is the speed of excavation. This is offset by potential coplanarity problems for the PCB which may cause damage to the PCB laminate or cause it to not be as effective, the cost of the specially tooled single use coupon, and the need to be able to program or stop the coupon from going down onto the PCB.

Whether using the solder wick approach, a solder vacuum excavation tool, or a wicking coupon, solder removal from the pads prior to placement of the replacement device is important from both a reliability and quality of placement perspective. The use of the technique depends on the kind of tooling capabilities, the sophistication of the rework equipment, and the experience of the operators. Cycle times and potential risks in damaging the PCB laminate factor into choosing the appropriate method for PCB rework.

This column originally appeared in the May 2021 issue of SMT007 Magazine

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2021

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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