Issue link: https://iconnect007.uberflip.com/i/1509873
48 PCB007 MAGAZINE I OCTOBER 2023 to make accurate measurements of insula- tion resistance. 19 Measurements of resistance above 10 12 are very difficult and require careful shielding. Measurements of resistance below 10 12 can be conducted in most laboratory envi- ronments if certain precautions are taken. e actual tests are usually conducted at elevated temperature and humidity with an applied DC bias. A test for moisture and insu- lation resistance of bare printed circuit boards is included in IPC-SM-840A. e severity of the test depends on the intended use envi- ronment; for typical commercial products (Class 2), the test is conducted at 50°C, 90 percent RH, and 100 Vdc bias for seven days. e minimum insulation resistance require- ment is 10 8 Ω. e military test procedure for moisture and insulation resistance is specified in PCB military specification, MIL-P-55110. 20 e moisture resistance test should be con- ducted in accordance with military specifica- tion, MIL-STd-202, Method 106, with applied polarization voltage (100 Vdc) and Method 402, Test condition A. IPC-SM-840A also includes a test for elec- tromigration resistance. e test is conducted at 85°C/90 percent RH at a 10 Vdc bias with a limiting current of 1 mA for seven days. A sig- nificant change in current constitutes a failure. e samples are also microscopically inspected for evidence of electrolytic metal migration. A common test for dendritic growth due to flux residues is 85°C/85 percent RH/1000 h at a -20 Vdc bias. ese tests are empirically based; however, several investigators have attempted to develop acceleration factors for these and similar tests. 21,22 More recently, accelerated reliability test- ing become an inevitable requirement, par- ticularly for high density interconnect (HDI) PCBs with microvias. IPC, as a leading PCB authority, has shied its focus toward perfor- mance-based acceptance testing, recommend- ing using test coupons to uncover latent micro- via failures—a departure from acceptance of reliability solely based on conventional micro- sectioning evaluations. IPC issued a warning since in a number of examples in high-profile hardware, failures were not manifest itself aer bare PCB fabrication, inspection, and accep- tance using thermally stressed coupons with traditional microsections and light micro- scopes alone. Data has been presented showing that con- ventional inspection techniques are no longer an effective quality assurance tool for detect- ing failures of microvia-to-target plating. So, it is critical to review the above acceleration test methods and tailor them for a product-specific environmental requirements, e.g., automotive and aerospace, in order to perform effective screening and life projections based on phys- ics of failure and underlying mechanisms. For detailed reliability guidelines, IPC's 9700 series Figure 2: Test coupons used to check moisture, insulation, and metal migration resistance: (a) the IPC-B-25 test board, used to qualify the process; (b) The Y coupon, designed to be incorporated into production boards for statistical process control. (Source: IPC-SM-840.)