Knocking Down the Bone Pile: BGA and PCB Warpage—What to Do

Warpage of BGA packages or PCBs can occur when any heating and subsequent cooling cycle is gone through. This may leave the package to either 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. If the PCB warps, then opens or shorts on a variety of other component areas may result.

Reasons for Problems

BGA and PCB warpage is a problem of material coefficient of thermal expansion (CTE) mismatch between the various packaging component materials, such as the substrate, silicon die, and EMC. The rate at which there are temperature increases can affect the temperature uniformity across the component during placement and removal, so this rate is indirectly correlated with warpage differences (Figure 1).

Wettermann_Jul2019-Fig1.jpg

Figure 1: Opens due to BGA warpage.

Also, the larger the package, the greater chance of this warping phenomena with all other things being equal. Certainly, the type of rework heating method (hot air rework system, IR, hot air reflow oven, vapor phase oven, etc.) also has an impact. Using low-CTE thermal materials, it is possible to tailor CTE, partially or fully eliminating this problem. Some plastic bodied ball grid arrays (PBGAs) include a heat spreader, which causes the top of the BGA package to expand at a faster rate than the bottom of the BGA; this can pull the corners of the BGA down. Moisture in the BGA may also contribute to warpage as the component wants to spread in the middle. In these cases, the corners may curl upwards.

Through a series of design of experiments, you may want to confirm which part of the equation—the BGA or the PCB—is warping. Isolating which surface is doing the pulling and pushing will be useful in determining how to fix the problem.

How to Mitigate

Wettermann_Jul2019-Fig2.jpg

Figure 2: Head-in-pillow defect.

For BGA warping, the corners will see the largest displacement, which can cause numerous opens and bridges to occur. Similarly, the circuit board can warp up or down and push into the solder paste, causing bridges or opens. These are caught either in visual or X-ray inspection. One of the methods to minimize warpage is to slow down your heating and cooling processes. You ramp up during your preheat section and cool down during your cooling section. Now, of course, in cooldown, you don’t want to go too slow because you don’t want to create a coarse-grained structure. As with many things in electronics manufacturing, it’s a trade-off.

Controlling MSD devices—including the boards and components—is another way to help mitigate the impact of warpage. The J-STD-0033 and JEDEC guidelines for moisture handling will be the best place to refer to for proper MSD handling. Prebaking of boards and components followed by keeping them in a dry environment will mitigate the problems if they are associated with moisture absorption. Limiting the exposure time and understanding the MSD level of the board and components will also go a long way to mitigating the warpage associated with moisture absorption. By using a solder paste that is formulated to reduce head-in-pillow and allows for the proper coalescing of the solder paste, solder ball warpage impacts will be limited as well (Figure 2).

Wettermann_Jul2019-Fig3.jpgFigure 3: Shadow moiré.

Through properly engineering the volume of solder paste deposited at each pad location, some of the problems related to PCB and device warpage can be limited. In some cases, overprinting some of the pads—while in other cases, underprinting other pads—may help compensate for the warpage impact. For example, where the BGA is bent inwards toward the PCB, there may be evidence of shorting. In those areas, the print volume may need to be minimized. Conversely, in areas where the BGA bends “away” from the board, a larger print volume may be the best solution.

If the board is warping (IPC-TM-650 2.4.22 test method for measuring bow and twist), then you may need to use other approaches to reduce the impact of device or board warpage (Figure 3). Besides the previously mentioned approaches, you may need to re-design the board to make sure the copper in the board is more evenly distributed. Low thermal mass (i.e., lower copper content) areas of the board will heat at different rates than higher thermal mass areas. In addition, very thin boards (0.020 inches) may need to laid out using a thicker board. Finally, different materials with closely matched CTEs may need to be used in the board or device construction.

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

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

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2019

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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Back

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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