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The use of VP reflow soldering technology can result in immediate yield improvements over convection reflow processes. For example, this contractor’s Mexico facility produces a PCBA which is also produced in China. The PCBA has microBGAs on both sides, including two microBGAs on the bottom side and seven microBGAs/BGAs on the top side. The largest BGA package has 266 balls. There is one chip scale package on each side of the PCBA. The 14-layer, high temperature FR4 OSP PCBA has 1809 components. In Mexico, VP reflow soldering is used, and convection reflow soldering is used in China. BGA opens/shorts are the number one issue in China, while the Mexico facility has experienced no solderability issues.
In another case, the facility is producing large PCBAs used in flat panel monitors. In convection reflow soldering a large PCBA can see as much as 90°C variance in temperature from one end of the PCBA to the other as it moves through various zones. If all parts are sitting in solder paste, this doesn’t cause an issue; however, if balls aren’t touching paste this thermal mismatch can cause parts to pitch slightly and form a gas boundary which causes an oxidized "pillow" to form. The end result is that parts touch but don’t reflow. In VP reflow soldering the difference in temperature from one end of the PCBA to the other doesn’t exceed 2°C, eliminating the potential for this issue to occur. In this particular project, the facility achieved yields of over 99% in the first three days of production ramp-up and the project is running approximately 30,000 PCBAs per month.
There are some tradeoffs that can be easily dealt with once a team becomes familiar with the process and the technology. One technical challenge with VP reflow soldering is that the process can exacerbate 0402 and smaller device tombstoning. The solution is to incorporate a reverse home plate design for those pads on the stencil.
Another issue is the efficiency of the vapor blanket. In the 1970s and 1980s, when VP was still a common option, one of the negatives associated with VP technology was the fact that hybrid parts could be heat damaged. Sealed parts with open cavities could expand or explode. DC to DC or AC to DC converters (hybrid potted modules) would often fail. Today’s technology minimizes those parts, but profile optimization is important because both sides of the PCBA go into reflow instantaneously.
A key selling point of the technology is that the process windows are broad enough that few unique profiles are needed. However, there can be a learning curve in developing the right profile because of the speed at which reflow begins. For example, the engineering team once ruined a profiler because the process was set at a level where the electronics inside the mole reflowed while taking measurements.
The Cost Savings Equation
VP reflow technology can save cost in several ways. First, it uses less energy than convection reflow soldering systems, in part because it doesn’t radiate excess heat. It not only uses less energy, but the factory also requires less energy for cooling the air near the machine. For example in Mexico, where summer temperatures routinely exceed 100°F, VP is 40% an hour less expensive than use of a nine-zone convection reflow oven just in machine energy consumption alone. Add to that the fact that energy costs in Mexico are 1.5x higher than those in the United States and the additional savings associated with less radiant heat, and the concomitant load that places on air conditioning utilization, add up quickly.
Figure 3: X-ray image showing consistent ball shape and solder wetting from vapor phase reflow.
One area that adds cost is the fluid used to generate the vapor blanket since it is a consumable. However, the cost of the fluid is near or below the cost of nitrogen frequently used in convection reflow ovens.
One other area where VP reflow soldering technology can save money is on mixed technology PCBAs. When there are only a few through-hole parts, the PCBAs can often be completely soldered using pin-in-paste and VP technology. Depending on the product mix, this can eliminate or reduce the need for selective soldering systems or wave soldering machines.
Additionally, while wave solder is typically a 3 Sigma process, SMT reflow is a 5 Sigma plus process, which means that the SMT process is much more repeatable and controllable in volume production.
VP reflow soldering technology has matured to the point where it can easily support higher volume production requirements. The challenge is properly sizing the machine to likely workloads and optimizing the necessary profiles. The end result is improved quality, better line flexibility and lower energy consumption.
Editor's Note: This article originally appeared in the January 2016 issue of SMT Magazine.