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

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. Many times, the first look or inspection of a reworked solder joint is the aesthetic appeal defined by the cleanliness inspection criteria found in IPC-A-610 (Section 10.6). Due to uneven heating from the rework process, corrosive no-clean flux activators may not be completely encapsulated with activated flux. This means the activators are on the exposed circuits and are likely to cause corrosion and dendritic growth, thereby negatively impacting reliability.

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In PCB rework, a no-clean flux (Figure 1) is active when spread out on the board and the only way that that residue becomes safe to leave on the board is when it is properly activated via a temperature reflow cycle. That requires all the flux, even that spilling over to neighboring devices and not being subject to a reflow cycle, to be cleaned off the PCB. Hand solder and rework methods that do not sufficiently control flux volume are going to leave dangerous residues such as chlorides and weak organic acids which may, in turn, impact the performance or reliability of the solder joints.

There are several ways to mitigate the problems associated with rework and the associated flux residues left behind:

Limit the volume of flux applied to the rework area. More flux means that there is a greater likelihood of non-benign residue being left on the board.

  1. Proper training on flux and flux residue removal for any hand assembly or rework technician. This training should include the method of cleaning, the method of rinsing, as well as the disposal of any remnant solution. The training on “when to use” what cleaning solution is also important.
  2. Proper training on the importance of activating the flux during rework.

Cleaning the flux post rework is defined in Procedure 2.2.1 of the IPC 7711/7721 “Rework and Repair of Printed Circuit Assemblies.” The procedure defines that up to 10 ml of cleaning solution for four square inches of affected area can be used to clean the reworked area. A clean, soft-bristled brush is then used to scrub the area followed by an isopropyl alcohol rinse. This is an important step as soiled brushes may bring unwanted debris and flux residue to the to-be-cleaned area. Any excess and leftover IPA from the rinsing process should then be cleaned with a lint-free cloth. Then the cleaning process inspection occurs per the agreed upon inspection criteria. Other cleaning processes can be used if agreed upon between the builder and the end customer.

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For PCB rework, even for a “no clean” flux formulation, the area around a solder joint area post reflow should be cleaned. The idea of a no residue or no-clean flux is a misnomer. Instead, the flux should be described as low residue. These residues, along with other contaminants from the rework area such as airborne contaminants, human skin, tool debris, etc. need to be removed. There are numerous reasons to clean an electronics assembly post rework, namely to:

  • Allow for better inspection/better cosmetic appearance around the solder joint
  • Promote the surface for better coating adhesion
  • Allow for testing to be done where required
  • Prevent electrical problems (noise or capacitive coupling)
  • Eliminate a longer-term reliability or corrosion problem

As PCB density increases, an understanding of the impact of cleaning becomes more challenging.  Premature failure or improper component functionality is site specific and, therefore, a localized testing means of cleanliness is crucial. A localized ionic contamination measuring method using an extraction technique in the area of interest has become more acceptable to measuring the contaminants left behind after PCB rework. Gone are the days of the ROSE testing which measures the ionic contaminants on the entire assembly.

There are several benefits to cleaning the low residue no-clean flux. One of the benefits is to ensure that any underfill or replacement conformal coating will be properly adhered to the PCB. Any residue left behind may outgas and impact adhesion. The right defluxing process and saponifier is used to facilitate better inspection and enhanced cosmetic appearance. A defluxing process is a cleaning process in that everything is cleaned. Contaminant species from board fabrication (like feeder tape residue), component fabrication residues, assembly residues, and human handling residues all are cleaned off in a cleaning process. All of these may cause performance issues and their resultant removal mitigates this risk.

While the cleaning of the flux holds some distinct advantages, there are several disadvantages to cleaning a no-clean flux after rework. Since DI water alone cannot be used to clean off the residue, a saponifier chemistry will need to be used and, therefore, extra expense added in terms of chemicals, cleaning equipment (if not already on site) as well as chemical disposal. By not properly cleaning all of this off, you can end up with active flux on the board and be in a worse position with respect to the reliability of the assembly. These and other drawbacks need to be considered when contemplating the necessity for a post-rework cleaning process.

The choice of the cleaning agent is important. It needs to match the residue and cleaning process. Dissolution of the residue at a fast rate is critical. Equally critical is the cleaning method’s ability to deliver the cleaning agent to the no-clean residues.

Reliable hardware is more challenging to build due to component size miniaturization, bottom termination devices, shortened distance between conductors, and higher pinout devices in a small footprint. These factors require that there is no entrapped flux residue and that the board is properly cleaned post rework.

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

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2021

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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