Issue link: https://iconnect007.uberflip.com/i/1243344
MAY 2020 I SMT007 MAGAZINE 89 needs to be performed only once during the entire life of that product. Developing a profile is like using correct DFM. You have to do it only once (DFM and profile development). But it is amazing to hear how the process engineers complain about DFM but don't take the time, barely an hour, to develop a unique profile for each product. Purpose and Types of Thermal Profiles What is the reason for developing a unique profile for each product? There is only one purpose for using the correct profile: to reduce defects and to produce acceptable electronic assemblies. Yes, there are many causes of defects, and the right profile is not the pana- cea for solving all defects. Nothing is because there are many causes of defects in an elec- tronic assembly, but the thermal profile plays a leading role. It is the simplest thing to take care of, but many people don't take the time. There are many ways to develop a thermal profile, but there is only one way to correctly develop the thermal profile, which I men- tioned earlier. There are two types of profiles: ramp to peak (RP) and ramp to soak to peak (RSP). The key difference between an RSP and an RP profile is the absence of the soak zone in the RP profile. An RSP profile allows a more uniform temper- ature across the PWBA, and it is very useful in achieving uniform temperature in a PWBA with a large variation in thermal masses of dif- ferent components. RSP profiles also make it easier to achieve lower voids in solder joints, especially in BGAs. RP profiles may increase the incidence of voids in solder joints, but they minimize the incidence of head-on-pillow in BGAs, which is a serious defect. Here is what you need to keep in mind. The presence of voids is not a serious concern for product reliability, but head-on-pil- low is an open causing a board to be nonfunc- tional; so it very simple. If there are BGAs on your board, worry about head-on-pillow and not voids. There are many causes of head-on- pillow—a topic that would take more than one column to discuss—but at least using the right profile is the easiest thing to take care of. Zones in a Profile There are only two things we control in developing a profile: belt speed and temper- ature settings in each zone. Further, there are four zones in any profile: preheat, soak, reflow, and cooling. Here is a brief summary of each zone. 1. Preheat Zone The temperature in the preheat zone can be from 30–175°C. Component suppliers recom- mend 2–4°C per second ramp rate to avoid thermal shock to sensitive components. Such guidelines are considered conservative since some capacitors are wave soldered and go from a preheat temperature of nearly 120°C to a wave pot temperature of 260°C. A high ramp rate increases the potential for solder balls, so keep it as low as feasible with consideration for the recommended ramp rate for the tem- perature-sensitive components on the board. 2. Soak Zone The soak zone is intended to raise the tem- perature of the entire PWBA to a uniform tem- perature. The recommended ramp rate in soak zone: 100–180°C for Sn-Pb and 140–220°C for SAC. The soak zone also acts as the flux activa- tion zone for solder paste. The consequences of having too high a temperature in the soak zone can include solder balls, solder splatter due to excessive oxidation of paste, and spent flux activation capability (burnt-out flux that fails to scrub the oxidized surface, its main function). Also, the purpose of a long soak zone is to minimize voids, especially in BGAs and BTCs. As mentioned previously, it is also common practice to not use a soak zone but to steadily ramp the temperature from the preheat zone to peak reflow. The likelihood of voids may increase when ramping steadily (RP profile) to peak reflow temperature. 3. Reflow Zone The peak temperature in the reflow zone should be high enough to obtain good wetting and to create a strong metallurgical bond. How- ever, the temperature should not be so high as