Issue link: https://iconnect007.uberflip.com/i/1540425
72 PCB007 MAGAZINE I OCTOBER 2025 Understanding Interconnect Defects, Part 1 by Michael Carano, C onsultant, Global Ele ctronics Asso ciation T RO U B L E I N YO U R TA N K This month, I'll address interconnect defects (ICDs). While this defect continues to rear its ugly head, don't despair. There are solutions, most of which center on process control and understanding the relationship of the chemistry, materials, and equip- ment. First, though, let's discuss ICDs. Interconnect Defects As circuit boards become more complex with mul- tiple layers, finer lines and spaces, and vias with smaller diameters and greater aspect ratios, the possibility of an interconnect defect looms large. If small hole voiding issues are not enough to keep everyone hopping, interconnect or interplane sepa- ration surely will. Unlike voids, where, if detected in time, the panels can be reprocessed, it is impossi- ble to rework the ICD defect. ICDs are not always detected by Quality Control., mainly because ICDs are rarely catastrophic failures. Rather, constant thermal cycling or the high temper- atures of assembly operations cause a weakened interconnect to separate. Many engineers expect that performing one or two solder floats on plated coupons removed from panels will provide a suffi- cient QC check. Unfortunately, this often allows the problem to go undetected until it is far too late. An example of an ICD is shown in Figure 1. A slight separation becomes detectable after the coupon undergoes two solder floats at 550°C for 10 seconds. If not seen by cross-section, the panel will easily pass electrical test. However, because of the weakness in the interconnect, continued thermal stresses and vibrations of the PWB will most likely lead to an open circuit. If you're unsure whether the defect is present, regrind and polish the pot- ted coupon, then inspect the ICD. Do not etch the specimen, otherwise, a line of demarcation can be visible with electroless copper deposits, which can lead you to conclude that it is a true ICD. It is imper- ative that un-etched coupons be used to detect the presence or absence of the defect. An ICD is not confined to just the interplane. Blind vias can also exhibit a separation of the plated copper from the capture pad. Intercon- nect defects (IP separation) generally exhibit these underlying characteristics: 1. Occurs during the stress of thermal shocks such as soldering. This creates expansion in the Z-axis and places strain on the copper in the hole wall. 2. Most often seen on the first inner layer from either side of the board. (i.e., on a 12-layer board, the defect can be seen on layers 2 and 11). 3. The weakest bond breaks first. 4. Under-cured boards are most susceptible. 5. Affected by board construction, PWBs with more resin, less copper, and lower Tg materi- als show more expansion in the Z-axis. 6. ICDs are often accompanied by a hole-wall pullaway, but not always. There are several causes of interplane separa- tion, making this defect so difficult to pinpoint and eliminate. To confuse the issue further, one must first determine which of the three types of IP it is. The three types of IP separation are: F i g u re 1 : E xa m p l e of a n I C D. ▼