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110 SMT007 MAGAZINE I MARCH 2019 After removing and replacing a ball grid ar- ray (BGA), the acceptability of the intercon- nection of the solder balls to the PCB should be assured. This assurance and the criteria for that assurance are the customer's outgo- ing inspection criteria. These acceptance cri- teria 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 Revi- sion C. Most of the requirements for the inspection of BGAs are based on IPC-A-610 standards. The current standard does not require X-ray inspec- tion. However, process validation can be used in lieu of X-ray inspection provided there is ob- jective 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 magnifica- tion is taken from the latest IPC-A-610 stan- dard. Solder terminations from the outside perimeter need to be visually inspected when- ever practical. By looking at the ball-board or ball-package interface, the degree of wetting, the amount of remnant flux residue or any ob- vious cracks, shorting or other anomalies can be noted. In addition, visual inspection will usually determine the proper pin "1" align- ment 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 sol- der connections, improper wetting, or lack of evidence of solder coalescing (if a paste print- based rework process is used). If X-ray inspec- tion is utilized for BGA inspection, a couple of the criteria to look for during BGA X-ray in- specting 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 (typical- ly tin-lead alloy) solder balls, the maximum al- lowable voiding allowable is 30% of the inspec- tion 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 non- collapsing balls, these voiding criteria are not established in IPC-A-610. As a reminder, the BGA may need to be re- worked 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 Inspection of BGAs After Rework Knocking Down the Bone Pile by Bob Wettermann, BEST INC. Figure 1: BGA X-ray image for inspection post-rework.