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March, 2020


www.us-tech.com


Using Continuous Vacuum Reflow Processing to Reduce Voiding


By Fred Dimock, Manager — Process Technology, BTU International, Inc. V


acuum reflow has been around for a long time with only limited interest. However, it is now gaining attention


in the electronics industry as it has been shown to be an effective method to reduce voiding in solder joints and thermal pads. Initially, vacuum reflow was a batch


operation where boards were placed in a vacuum chamber and heated to melt the solder. During the heating process, the chamber was evacuated and the void level was reduced or, in some cases, removed from the solder joint. Occasionally, vacuum was


applied when the solder approached the liquidus and at other times vacu- um was applied during the complete heating cycle. This equipment was expensive to purchase and operate, though the biggest drawback was low throughput.


Therefore, vacuum reflow was


not appropriate for high-volume pro- duction. In addition, there were ques- tions about the need to eliminate voids, with some claiming that a small number of voids in joints actu- ally increased their strength. There was little motivation to


pursue vacuum reflow until the need to dissipate heat from high-power components (especially automotive under-hood components) arose. At first, vias seemed to be part of the answer but it was impossible to guar- antee that a via would be under the “hot spot” in the component, there- fore low or no voids in the thermal pads seemed to be the answer.


Defining Low Voiding “Low voiding” can mean any-


thing from 20 percent to 0, with many people specifying 5 percent as the desired realistic limit. The challenge has been to reduce 40 percent voiding to 20 percent or less, with high-volume production equipment. Solder paste and thermal profile modifications have helped, but consistently getting below 5 percent is a stretch. One solution is a continuous


vacuum-assisted reflow oven, where a vacuum chamber is placed between the last heated zone and the cooler, in a standard convection reflow oven. This presents material and engineer- ing design challenges, because the board has to be transported into and out of the chamber, the vacuum sec- tion has to seal and the vacuum must be applied at elevated temperatures. There are many designs cur-


rently being used for the internal vac- uum chamber that use entrance and exit doors or bell jar configurations. Doors tend to be lightweight and move easily, while the bell jar design makes access to the conveyor system inside the chamber simple. The transport system presents


issues, since the edge rails have to be interrupted for the door or bell to seal and the board must stop in the cham- ber while vacuum is being applied. In each case, the seal material must withstand temperatures that ap - proach 662°F (350°C).


Continuous Vacuum Reflow The initial heating of continuous


vacuum reflow is exactly like a normal reflow oven, where convection rate, zone set points and belt speed are the factors in heating the board.


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    


MLF 100 X-ray images showing voiding after reflow with no vacuum (left), 120 Torr (center) and 20 Torr (right).


Once the board begins to enter the


vacuum chamber the power of convection to heat the board is lost and radiation must be relied upon. When the board is in the vacuum chamber, four things are con- trolled: pump down rate, vacuum level, vacuum hold time, and equalization rate. Pump down rate is the speed at


which vacuum is applied. If it is too fast, the voids can explode as they leave the joint or pad and produce solder balls or


Continued on page 72


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