Knocking Down the Bone Pile: Humidification for ESD Control in PCB Rework/Repair

The amount of charge generated in an electronics rework and repair area is affected by a variety of factors, including but not limited to, the materials used and the amount of frictional interaction between materials, as well as the relative humidity of the environment. During the cold winter months in northern climates when the heating systems dry out the plant air and the relative humidity falls, higher electrostatic charges develop, all other things being equal. Lower humidity can increase the number of ESD events, so theoretically it stands to reason that keeping the rework area at higher humidity levels will reduce the chances for charged-induced damage to components.

To get to the “right” relative humidity level in the PCB rework/repair area, several variables need to be taken into consideration. The electronic components being reworked need to fall within their specified RH operating range. In addition, the rework processing steps, such as the length of time the repair epoxy takes to cure or the amount of time the conformal coating material needs to cure, are some of the process steps which will be impacted by the humidity level. Too high of a relative humidity level may result in unwanted quality problems such as corrosion, hand soldering defects, and unwanted MSD damage to moisture sensitive devices. Solder paste will not have the right printing and slump properties at higher relative humidity levels. This may impact paste printing rework processes such as leadless device or BGA component site printing. Typical relative humidity ranges for the PCB rework and repair area are between 30-60%. With a 40% relative humidity level, surface resistance is lowered on floors, carpets, table mats, and other susceptible areas (Figure 1).

Wettermann_Jan_Fig1_cap.jpgIncreased humidity levels can be accomplished through humidification systems. Humidifiers add water vapor to the air, which forms a thin protective film on surfaces and serves as a natural conductor to dissipate electrostatic charges. When humidity levels drop below 40% RH, this protection disappears, thereby increasing the possibility of damage or defects within electronic components and devices.

Wettermann_Jan_Fig2_cap.jpgThere are numerous risks to operating the PCB rework and repair area when the air is at low relative humidity. If any existing static monitoring or control systems fail (e.g., a ground connection becomes disconnected, operators missing a wrist strap, foot grounders, or grounding mats will have had coating spilled on them making them insulating surfaces), there is no backup to controlling static charging impacts. Secondly, any reworked board which is not touching an ESD-safe surface or handled properly by an ESD-protected rework technician can be damaged. In many cases, controlling the humidity levels is easier than ensuring that non-charge-generating material does not enter the workspace. These are some of the risks to operating the rework area in a low humidity environment.

Adding humidity to the rework area cannot replace a robust ESD control system just by employing humidification, rather it may be used in conjunction with such a control program. Such a program should contain the following elements:

  • A training/retraining program of PCB rework/repair technicians
  • Operators wearing wrist straps when handling ESD product
  • A static dissipative floor
  • All ESD benches identified as such and properly connected, each with its own connection to ground
  • Operators wearing dissipative footwear
  • Operators wearing ESD-safe smocks correctly
  • Charting/monitoring of the relative humidity along with monitoring alarms
  • Keeping insulators out of the rework/repair operating zone per EOS/ESD 2020 requirements
  • An audit system in place to monitor all the above, with sufficient frequency for each method to ensure compliance and effectiveness

Air humidifiers cannot replace these and other ESD control measures. Adding moisture to an environment where two materials of differing electrostatic potential come into contact and separate (like Kapton™ tape being pulled off a dispenser) will not stop the charging from occurring. However, adding moisture to the environment will reduce the number of ESD events. No humidity control system will prevent this.

Commercially available industrial humidification systems have a variety of features as part of their design. The systems designed for electronics assembly are typically closed-loop systems where moisture is measured, and the level is controlled. The water used passes through hygienic multi-stage filtering systems including UV lighting. This helps ensure that the water vapor being dispensed does not damage machinery and electronic assemblies. Many systems have line flushers to purge out impurities from time to time.

At the end of the day, in the deep of winter in northern climates, the heating systems employed in the PCB rework area will dry out the air and move the relative humidity to low levels. This in turn will allow ESD events to occur more readily, putting pressure on the robustness of the ESD control and monitoring program. While humidification systems are not required in all cases, their use in certain conditions may be warranted.

References

  1. “Coaxing better performance from electrostatics demonstrations in humid conditions,” Thomas Jones, class notes, University of Rochester.

This column originally appeared in the January 2022 issue of SMT007 Magazine.

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2022

Knocking Down the Bone Pile: Humidification for ESD Control in PCB Rework/Repair

01-05-2022

The amount of charge generated in an electronics rework and repair area is affected by a variety of factors including but not limited to the materials used, the amount of frictional interaction between materials as well as the relative humidity of the environment. During the cold winter months in northern climates when the heating systems dry out the plant air and the relative humidity falls, higher electrostatic charges develop all other things being equal. Lower humidity can increase the number of ESD events so theoretically it stands to reasons that keeping the rework area at higher humidity levels will reduce the chances for charged-induced damage to components.

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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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