Issue link: https://iconnect007.uberflip.com/i/992528
74 PCB007 MAGAZINE I JUNE 2018 chemistry/material suppliers to make improve- ments in the fabrication process to enhance long-term reliability. These improvements are not exclusive to process and materials as de- sign of the PWB may also contribute to failure. IST has both the capability of effectively and rapidly quantifying the integrity of the plated through-hole (PTH) and the unique ability to identify the presence and levels of post sepa- ration within the multilayer board (MLB) [1] . A typical IST coupon has two circuits typically designed into the coupon: a power circuit for direct heating through the interconnect and a sensing circuit to measure stress and strain in the barrel of the PTH. This test method measures cycles to fail- ure on these specially designed IST coupons that are thermally cycled using current flow through internally heated circuits that heat the coupon and the adjacent plated-through holes. Failure is defined as a 10% increase in the re- sistance of the PTH [2] . In short, there are any number of factors that can precipitate the failure of either the electro- less copper-to-post interconnect, or the failure of the electroplated copper in the via itself. In terms of failure in the plated-through hole or the post interconnect, thermal excursions are the major cause of failures. Figure 1 depicts a schematic of key defects in the plated through- hole that are thermally induced. The primary cause of barrel cracking is the CTE mismatch between the resin matrix and the electrodeposited copper. This is taking place in the Z-axis. Movement in the X-Y plane is somewhat more restricted than in the Z-ax- is due to the glass fiber reinforcement. In addi- tion, the resin CTE increases sharply above its glass transition temperature, causing further CTE mismatch strain induction on the copper and the potential for PTH/PTV failure. With all this information, where does the fabricator go from here? Certainly, using high- er Tg, lower CTE materials would be a bene- fit for improved reliability. But at what added material cost? In addition, a change in material will certainly impact other aspects of the board including electrical functionality that may not be acceptable to the end user. Interconnect defects (ICD)—that is, the sep- aration of the plated copper (either electroless or electrolytic) from the post innerlayer—is a different failure mechanism than described above. The good news is that IST testing can differentiate between barrel cracking failures and ICD failures. The bad news is these are both failures. However, understanding what the actual failure is will help guide the trouble- shooter to get to the root cause of the defect. So, assuming the material set is as is and the board construction is as is, let's first describe the defect and work through the troubleshoot- ing process. Zeroing in on the Problem As described earlier, several boards failed due to an open detected after solder float. While the operations team wanted to blame the new- ly installed PTH process, closer inspection of the problem showed that the defect was barrel cracking, not a separation or any other defect related to electroless copper. To study the issue further, the fabricator decided to use IST and Figure 1: Common failure mechanisms in plated through- holes. (Source: IPC Wisdom Wednesday presentation, July 2017)