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

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.

Hand Soldering Rework

Hand soldering of high lead count or very small package SMT components for rework is fast and requires a high degree of skill from the operator. The good news about using the hand soldering process for these types of SMT components is that a device during debug can be quickly reworked and passed back to test or design engineering.

Wettermann-100919-Fig1.jpgFigure 1: Hand soldering for rework of fine-pitched components.

However, there are some deficiencies in using a hand soldering rework process for electronic components. First, the lack of consistent solder volumes and consistent hand soldering uniformity makes this an uncontrolled process. When the process allows for adding flux, the lack of control over this flux volume means that flux residue and board cleanliness can be a potential reliability risk. For some of the fine-pitched parts in high-density areas, a high degree of rework technician skill is required, which limits the throughput (Figure 1) In addition, nearby components can become damaged if the technician is not careful when placing the soldering iron. Also, hand soldering is not a cost-effective option in some cases, as it can be very time-consuming.

Stencil Printing

Stencil printing for the rework of high pin count or very small package SMT components for PCB rework has some pluses and minuses. The advantages to using stencil printing for the rework of these devices are numerous and include this being a well-known technique with well-known materials, requiring a moderate degree of skill from the rework technician. The speed of the rework method is also fast compared to the other methods.

Wettermann-100919-Fig2.jpgFigure 2: Flexible stencil paste print.

The flexible, single-use stencils (Figure 2), while still being space-constrained, allow the rework technician to get into tight spaces. These adhesive-backed stencils have overcome the limitations of the metal stencils, including board coplanarity, the bending of the stencils during handling, getting into tight spaces, the necessity to tape around the device to prevent solder paste from contaminating the area, and the necessity for a holding fixture for alignment and time it takes to clean the stencils.

But this rework method has drawbacks, including the handling of stencils when very small packages are involved, the “messiness” related to solder paste printing, and the time it takes to have the stencils on hand (24–48 hours) all limit the usefulness of stencil printing solder paste for rework.

Dispensing

Dispensing, along with jet printing, offers an alternative to both stencil printing as well as hand soldering. There are several advantages for dispensing when it comes to the rework of high I/O count and/or very small package components (Figure 3). The biggest differentiator of this technology is the precision of the dispensing technique. Modern Archimedes screw-type dispensers can dispense 20,000 dots per hour down to 800-um sizes. A single jet printing setup can print at a constant speed of up to 300 Hz, or 1,000,000 dots per hour, with the dots getting down to 200-μm in size.

Wettermann-100919-Fig3.jpgFigure 3: Dispensed solder paste on ultra-fine-pitch SMT component.

One of the other big advantages of jetting is that multilevel boards and odd-shaped lands—such as RF shields with the same deposited solder volume—offer high repeatability. In addition, this method is more automated than the stencil or hand solder rework methods. While this method offers many advantages, it is not for every scenario due to its capital-intensive nature, the programming required, and the relatively slow speed of deposition. Dispensers that can be used for solder paste deposition start at several thousand dollars and range up to over $100,000 U.S. dollars. And each of the package footprints take time and skill level to program.

This method, even when finely tuned, is very slow compared to the methods. Finally, any process that involves a liquid and dispensing like this requires expertise so that lines, nozzles, and other elements of the dispensing system are kept clean and optimized. This is rheological “tightrope to walk” with dispensing technology and the associated precision deposition upside.

Conclusion

The process engineer has to take each rework situation and consider the turn time required, the reliability of the assembly in the end-use operating environment, the skill level of the operators, the available funds, and the economics to make the right decision.

Bob Wettermann is the principal of BEST Inc., a contract rework and repair facility in Chicago.

This article was originally published in the September 2019 issue of SMT007 Magazine.

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2019

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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