Initial heating is exactly like a normal reflow oven, where convection rate, zone set points and belt speed are the control factors in heating the board. This brings us to the process steps encountered in a continuous vacuum reflow system. In each case the seal material must withstand temperatures approaching 350☌.įigure 1. The transport system presents issues because the edge rails must be interrupted for the door or bell to seal, and the board must stop in the chamber while the vacuum is applied. Doors tend to be lightweight and move easily, while the bell jar design makes access to the conveyor system inside the chamber simple. Numerous designs currently used for the internal vacuum chamber employ entrance and exit doors or bell jar configurations (FIGURE 1). the vacuum section must seal, and vacuum must be applied – all at elevated temperatures. This presents material and engineering design challenges because the board must be transported in and out of the chamber. The answer is a continuous vacuum-assisted reflow oven, in which a vacuum chamber is placed between the last heated zone and the cooler in a standard convection reflow oven. The question became how do we get from the current 40%+ voids in thermal pads to 20% and below with high-volume production equipment? Solder paste and thermal profile modifications have helped, but void levels consistently below 5% are a stretch. Numerous engineers say 5% is the desired realistic limit. Today “low voids” can mean anything from 20% to zero. Therefore, low or no voids in the thermal pads seems the answer. At first, vias seemed part of the answer, but it was impossible to guarantee a via would be under the “hot spot” in the component. There was little motivation to pursue vacuum reflow until the need to dissipate heat from high-power components came on the scene, especially automotive under-the-hood components. Additionally, there were questions about the need to eliminate voids, with some claiming a small number of voids in joints actually increases their apparent strength. Therefore, vacuum reflow was not appropriate for high-volume production. The equipment was expensive to purchase and operate, but the biggest detriment was throughput. Sometimes the vacuum was applied when solder approached liquidus, and at other times vacuum was applied during the entire heating cycle. During the heating process the chamber was evacuated and the void level was reduced or, in some cases, removed from the solder joint. Initially, vacuum reflow was a batch operation in which boards were placed in a vacuum chamber and heated to melt the solder. ![]() Vacuum reflow has been around for some time with only slight interest but is now gaining attention in electronics assembly because of a desire to lower and even eliminate voids in solder joints and thermal pads. MLF trials show relatively soft vacuum levels and short hold times can significantly lower voiding.
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