Inspection of BGAs After Rework

After removing and replacing a ball grid array (BGA), the acceptability of the interconnection of the solder balls to the PCB should be assured. This assurance and the criteria for that assurance are the customer’s outgoing inspection criteria. These acceptance criteria are outlined in a very general fashion in the latest version of IPC-A-610—Acceptability of Electronic Assemblies, Revision G as of this writing—based on the class of the assembly. Other approaches to the acceptance criteria can be found by augmenting the requirements of 610 guidelines with your own criteria based on testing outcomes or the suggestions found in IPC-7095—Design and Assembly Process Implementation for BGAs, currently at Revision C.

Wettermann-Fig1-Mar20.jpgFigure 1: BGA X-ray image for inspection post-rework.

Most of the requirements for the inspection of BGAs are based on IPC-A-610 standards. The current standard does not require X-ray inspection. However, process validation can be used in lieu of X-ray inspection provided there is objective evidence of compliance. When visual inspection is done on a BGA post-rework, and the requirements of IPC-A-610 are the basis for inspection, the proper inspection magnification is taken from the latest IPC-A-610 standard. Solder terminations from the outside perimeter need to be visually inspected whenever practical. By looking at the ball-board or ball-package interface, the degree of wetting, the amount of remnant flux residue or any obvious cracks, shorting or other anomalies can be noted. In addition, visual inspection will usually determine the proper pin “1” alignment of the component body.

Other visual indications include missing solder balls, minimum electrical clearance distance to neighboring conductors, head-in-pillow defects, solder bridging, fractured solder connections, improper wetting, or lack of evidence of solder coalescing (if a paste print-based rework process is used). If X-ray inspection is utilized for BGA inspection, a couple of the criteria to look for during BGA X-ray inspecting taking place need to be reviewed.

If X-ray inspection is used post-BGA rework, then there are a couple of criteria called for in the IPC-A-610 standard. For collapsing (typically tin-lead alloy) solder balls, the maximum allowable voiding allowable is 30% of the inspection area. What is excluded from this amount are plating process-related voids, which should be established between the end customer and the company performing the rework. For noncollapsing balls, these voiding criteria are not established in IPC-A-610.

As a reminder, the BGA may need to be reworked for a variety of reasons. Some of the reasons for BGA rework include a defective component, an upgrade in memory size, or a change in a revision of the part. After removal and replacement of the component, inspection of the newly placed component—whether it is a new component or a reballed component—needs to take place to ensure the replacement components’ interconnections.

X-ray inspection of BGAs post rework (Figure 1) require inspection time to determine the acceptability of the solder interconnection. A “scan” of the BGA area from a far enough field of view to see the entire array is a good way to begin the BGA post rework inspection process. After this far field of view scan, the technician should zoom in to get a good visual indication at close range of the ball shape, size, and consistency.

Wettermann-Fig2-Mar20.jpg

 

Figure 2: Automated BGA X-ray inspection post-rework can catch a missing ball.

Starting at one corner, the technician manipulates the X-ray back and forth over the array until all of the balls have been scanned. More advanced X-ray systems can be programmed with acceptance/reject criteria to more fully automate this process. More automated X-ray systems can scan for concentricity (i.e., ball shape), ball diameter, pitch, and other parameters (Figure 2).

Summary

Many of today’s designs include a BGA or area array package. When this component needs to be swapped out for rework, the inspection of the replaced BGA may include both visual as well as X-ray inspection. Following the customer agreed-upon inspection criteria, there may be visual, basic X-ray inspection, or more advanced automated inspection of the reworked location may need to be made to make a go/no-go determination of the interconnection.

Bob Wettermann is the principal of BEST Inc., a contract rework and repair facility in Chicago. To read past columns or contact Wettermann, click here.

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2019

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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