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

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. 

There are some common board layout or design challenges which can lead to improper or inconsistent hole fill for those PCBs with high thermal mass. When plated through-holes are connected to ground planes or higher-weight innerlayer copper, the thermal mass of that component lead is higher than surrounding ones. This means that to get the board up to the correct reflow temperature requires a longer period of time. If the lead/hole is not brought up to the proper temperature, the wettability of the solder in the hole is lower, thereby not allowing the solder to flow all the way up the barrel.

In other layouts there may be an uneven distribution of thermal mass to each of the component leads which may then have the same impact on hole fill. Another design challenge comes into play when more temperature-sensitive devices are in close proximity to the through-hole devices thereby causing the process engineer to “dial back” the temperature during wave soldering preheat in order to not damage the component. This lower temperature can cause the same insufficient lower hole fill. These are just some of the implications of the design and layout challenges on the hole fill.

C_Wetterman_Fig1.jpgNot only can board layout and design challenges cause inconsistent or poor hole fill but assembly processing problems may result in the same deficiency. A recent work [1] indicated that choosing the proper flux was one of the biggest influences on hole fill during wave soldering. A second large influence of improper hole fill is poor penetration of the flux into the through-hole. Another source of improper hole fill is improper board preheating and a lack of dwell time in the wave soldering source. Too long a dwell time can lead to copper dissolution of the plated holes (Figure 1). If pin-in-paste solder printing is used in lieu of wave or selective soldering, insufficient solder paste volume or the location of the print may be cause for improper hole fill. These and other process problems are some of the more common causes of process-related hole fill deficiencies.

Discovering improper hole fill requires that the proper inspection protocols are in place in order to detect a defect. Per the IPC A-610 standards, there are a couple of ways to inspect for proper hole fill. Visual inspection can infer hole fill, but only if substantiated by other means through a process capability analysis. For example, if there is circumferential wetting on both source and destination sides of the plated hole, it can be inferred that the hole is filled. The process for assembly can be optimized and hole fill measured by cross-sectioning the plated through-hole. The process can be confirmed to be in control by linking this hole fill cross-section amount to the visual inspection of both source and destination leads and lands along with the appropriate confidence interval.

Otherwise, x-ray inspection (Figure 2) can confirm hole fill percentage by using built-in algorithms. While AOI systems can detect the presence of both source and destination fillets they cannot determine positively that there is solder in the barrel.

C_Wetterman_Fig2.jpgOnce the defect is discovered and it has been decided that the component should be reworked as part of the boards’ disposition, there are several rework processes which can be employed. Adding additional solder to the plated through-hole can take on several different forms. A solder fountain with the proper preheater along with the proper flux application can, in some cases, fill the remainder of the barrel. Care must be taken to ensure that the dwell time above the soldering source is kept to a minimum, especially as copper dissolution may result in thinning of barrel walls or incomplete knees in the barrel. Heating of the board in an oven prior to being subjected to the wave temperatures reduces thermal shock to the assembly while limiting dwell time. In the simplest of cases a soldering technician along with the proper flux and board preheater can solve the insufficient hole fill problem via flux and a soldering iron. In cases where the boards have a large thermal mass, the board may first have to be preheated in a controlled oven. Immediately upon removal from the oven the flux and solder can be added to the hole to fill the hole and meet the specification.

Through careful process troubleshooting along with an understanding how board layout can result in insufficient hole fill, the proper rework process can be chosen in order bring the barrel fill into compliance.

 Resources

Thomas Shoaf, Joseph Clure, Denis Jean, “Achieving excellent vertical hole fill on thermally challenging boards using selective soldering,” SMTAI Proceedings 2009.

  1. IPC-A-610 Rev H 7.3.5.1 Supported Hole Solder Fill (a).
  2. C Hamilton “A Study of Copper Dissolution During Lead-Free PTH Rework,” Circuits Assembly, May 2006.
  3. Brian Czaplicki, “Advanced Through-Hole Rework of Thermally Challenging Components/Assemblies: An Evolutionary Process,”  IPC APEX 2009.

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

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2021

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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