Issue link: https://iconnect007.uberflip.com/i/1526666
48 PCB007 MAGAZINE I SEPTEMBER 2024 Why Do ICDs Often Go Undetected? Figure 1 depicts an example of an ICD sep- aration. One can detect a slight separation aer the coupon was subjected to two solder floats at 550°C for 10 seconds. If not seen by section, the panel would easily have passed the electrical test. However, because of the weak- ness evident in the interconnect, continued thermal stresses and vibration of the PCB will most likely lead to an open. If in doubt about whether the defect is present, simply regrind and polish the potted coupon and inspect. Remember, inspect for ICD only aer polish- ing the coupon. Do not etch the specimen. When a specimen is etched, a line of demar- cation is oen visible with electroless copper deposits. is line may lead one to conclude that it is a true ICD. Un-etched coupons must be used to detect the presence or absence of the defect. Certainly, an ICD is not confined to just the IP. Blind vias can also exhibit a separation of the plated copper from the capture pad. ICD (IP separation) generally exhibits these underlying characteristics: 1. Occurs during the stress of thermal shocks such as soldering. is produces expansion in the Z-axis and places strain on the copper in the hole wall. 2. Most oen seen on the first inner layer from either side of the board. (i.e., in a 12-layer board, the defect is seen on layers 2 and 11). 3. e weakest bond breaks first. 4. Under-cured boards are the most susceptible. 5. Affected by board construction, PCBs with more resin, less copper, and lower Tg materials tend to show more expansion in the Z-axis. 6. ICDs are oen accompanied by hole wall pullaway, but not always. ere are many causes for IP separation. is is what makes this defect so difficult to pinpoint and eliminate. To further confuse the issue, one must first determine which of the three types of IP it is. ICD Root Cause Now, let's dive into the root causes of the three primary types of ICDs. Figure 2 depicts a Type 1 ICD. Type 1 is the most common. Since this defect involves so many processes and additional insight i nto the desmear/electroless copper process, I will defer the discussion on Type 1 to a future column. Figure 3 shows a different type of ICD. Here, the electroless copper remains on the post or the interconnect. So, it's not a Type 1. Instead, the separation is between the electrolytic cop- Figure 1: An example of an ICD separation. Figure 2: Type 1 ICD: Electroless copper separation from the post.