SMT007 Magazine

SMT007-Jan2019

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80 SMT007 MAGAZINE I JANUARY 2019 The cooling zone helps determine the integ- rity of the solder joint grain structure. A quicker cool down ramp, in comparison to the preheat phase ramp, is normally desired but take care not to exceed the CTE of components and board surface. A common recommenda- tion for cooling ramp rate is no more than 4°C per second. More detailed information can be found in the IPC-7530: Guideline for Temperature Profil- ing for Mass Soldering (Reflow and Wave) Processes. Profiling Equipment There are several choices of reflow profiling equipment available depending on the need. There are profilers for either the product or the reflow oven. For this study, I will focus on only the product profiling equipment that travels with the product, thus eliminating the need for long wires that run the length of the oven. The product profiler should be capable of measur- ing multiple locations on the assembly. Most commercially available traveling type profilers have up to six separate thermocouples. Some have real-time measurements sent to a receiver on a computer display, and others use inter- nal memory to store the data points for down- loading after the product exits the reflow oven. Either type can yield the desired analysis. Design of Experiment Multiple thermal profiles were used for this experiment to determine what effect tempera- ture has on cleanliness and electrical resistance measurements. Cleanliness was measured using ion chromatography (IC) and surface insulation resistance (SIR) testing is performed in an environment of 40°C and 90% relative humidity with 5V bias with measurements made every 10 minutes. All test boards are the Umpire two qualification test board (Figure 2). The first thermal profile was 20° below the recommended limit. The second profile group was done at 10° below the recommended limit. Meanwhile, the third profile group was done at the manufacturer's recommended profile for the solder paste. This is what is considered the minimum allowable temperature for proper solder joint formation and full flux activation. The fourth profile was done at 10° above the recommended limit. Ten boards were processed for each profile group, and of the 10, five were tested with IC; the remaining five from each group were subjected to SIR testing. Each board was measured at four different locations including an LCC, TQFP, BGA, and a non-populated row of headers for reference. Analysis Techniques As previously mentioned, the experiments involved two kinds of analysis techniques: IC and SIR testing. All IC testing was performed using a Dionex ICS 3000 chromatography system with Chrome- leon software. The extractions were performed using an automated localized extraction Figure 2: Umpire two test board.

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