Issue link: https://iconnect007.uberflip.com/i/550232
August 2015 • SMT Magazine 51 mum warpage at temperature of 0.025 mm (1 mil). In comparison, the BGA had more than 0.090 mm (3.5 mil) of warpage. The authors considered the PCB to be very flat and stable during reflow and unlikely to be a main con- tributor of HoP defects . Observations The following are the key observations stem- ming from these case studies: 1. All of the cases meet one or both industry specifications governing maximum acceptable warpage. 2. Each of these cases studies resulted in HoP defects on the assembly. 3. The onset of HoP occurs at component warpage values in the range of 0.075–0.090 mm (3-4 mil) during reflow as suggested by these cases. 4. The higher the warpage is for the same BGA package, the higher the incidence of HoP defects. 5. For BGAs of 0.8 mm pitch and above, the case studies did not show a strong correlation between degree of warpage and BGA size. 6. The case studies that have supporting PCB data suggested that the PCB warpage is not a key contributor to the HoP defect. Solder Paste volume Theory A simple theory of solder paste volume is used together with the examples provided to support the propose acceptance criteria. Most solder pastes in use contain roughly 50% flux by volume. Therefore with a 0.150 mm (6 mil) stencil, once the flux content is burned off, the theoretical height of the remaining solder is 0.075 mm (3 mil). At liquidus temperatures, the solder domes up slightly similar to stage (d) de- scribed in Figure 2. If the component has pulled away from the solder paste before liqudus, then the oxides and contaminates on the solder ball cannot be reduced by the flux. This theory sug- gests that if the solder paste does not remain in contact with the solder ball at liqudus, poor or no contact will result, thereby generating HoP defects. Table 3: Warpage measurement of pcb for case study 5. Table 4: Warpage measurement result of the pcb site for case study 6. WARPAGE ACCEPTANCE PROPOSAL continues FeAture