Issue link: https://iconnect007.uberflip.com/i/1545666
60 SMT007 MAGAZINE I JULY 2026 • Endoscope inspection: Used to view some inner rows of a BGA for wetting and align- ment otherwise inaccessible by an optical mi- croscope • X-ray inspection: Used to inspect internal sol- der joints, detect voids within solder balls, and identify bridging between solder balls that cannot be seen visually • Electrical testing: Functional testing to ensure BGA connectivity, but requires specialized equipment and skilled technicians, and is a time-consuming process While optical microscopes and endoscopes for visual inspection are useful as an initial check of a BGA assembly, they are limited in the ability to detect other critical parameters, such as voiding and solder bridging. Acceptance Criteria The key acceptance criteria for BGA assembly per the IPC-A-610 and J-STD-001 standards include: • Alignment: Solder balls are centered and show no offset of the ball-to-land centers or spacing violations • Solder ball spacing: Solder balls do not violate the minimum electrical clearance • Solder connections: BGA solder balls con- tact and wet to the land, forming a continuous elliptical round connection; no solder bridg- ing, opens, missing balls, or unsoldered con- nections • Solder ball uniformity: While not considered a defect, solder balls that are not uniform in size, shape, coloration, and color contrast are considered a process indicator for Class 2 and Class 3 • Solder voids: 30% or less of any ball in the X-ray image area • Solder ball appearance: Solder balls should show evidence of proper reflow, wetting, and uniform collapse across the array • Head-in-pillow: No head-in-pillow defects • Foreign object debris (FOD): No signs of conductive or loose debris that violates clearance or can affect function under or around BGA Popcorning The term "popcorning" refers to a failure mode associated with moisture ingress into surface mount components such as BGAs. If moisture-sensitive devices, including BGAs, are not retained in a nitrogen-dry storage cabinet, they are susceptible to the popcorn phenomenon. Popcorning occurs when a small amount of moisture trapped within the polymer molding of a BGA is converted into a large volume of steam during the solder reflow process. This causes the BGA package to expand like a kernel of cooked popcorn, potentially leading to cracks and field failures. Popcorning of BGAs can be indicated by bridging between adjacent solder balls beneath the device, since the bottom of a BGA is its thinnest point. This occurs when the package expands during solder reflow, causing it to "dish" downward; the under- side of the package deforms and presses down on the solder balls. As the solder is liquid at the time, it allows solder from adjacent balls to coalesce, thereby forming a solder bridge underneath the BGA device. The most typical cause of popcorning is the hygroscopic sensitivity of the molding compound used to protect the die. Device manufacturers K N O C K I N G D OW N T H E B O N E P I L E Figure 1: Head-in-pillow BGA defect as detected by X-ray inspection.