Vapor Phase Technology is a Viable Solution, but Carries a Learning Curve
Talk to electronics manufacturers about vapor phase (VP) reflow solder technology and you'll find people who either love it or hate it. The reason for this diversity of opinion is due to fact that it is a technology that many people still do not fully understand. Yet, it is also a technology that has come of age in an era where its efficiency in reflowing densely packed printed circuit board assemblies is highly valued, since the vapor blanket immersion process ensures perfect wetting and void-free, high-quality solder joints.
Firstronic's electronics contract manufacturing operation in Juarez, Mexico has installed two VP reflow soldering systems over the last year. The facility also uses convection reflow soldering.
Figure 1: Firstronic has added two vapor phase reflow soldering systems to its Juarez, Mexico facility.
On the positive side of the equation, VP reflow soldering offers several advantages:
• Fewer process windows, which reduces changeover time
• Cleaner solder joints at lower temperatures
• More even heating for even large PCBAs
• Can eliminate need for wave solder or selective solder on mixed technology PCBAs
• Lower energy costs than convection reflow technology.
However, there can also be learning curve issues and tradeoffs:
• While the oven is in line and operates as a pass-through system, VP technology is still a batch process and the machine capacity must be sized to projected line volume
• The efficiency of the VP process can generate surprises in initial process development
• The fluid utilized in the process is a consumable which adds to cost.
An Excellent Solution for Lean Manufacturing
One of the big advantages in VP reflow soldering technology is that relatively few unique profiles are required. There is zero wait state between products and VP requires far fewer profiles than found in convection reflow soldering. Comparatively, convection reflow oven stabilization can add as much as 15 minutes to a changeover process.
That said, VP reflow is a batch process and throughput can't be changed. While that works well with the smaller lot size focus of Lean, the system can become a constraint if sized below the likely line volume. For example, Firstronic selected an IBL CX600 VP reflow soldering system when it added the technology in November 2014 because the facility was ramping production and a conservative approach seemed justified. The machine is sized for medium- to high-volume production environments, which typically would have no more than two SMT placement machines feeding the reflow process. When the second system was added in mid-2015, a higher volume CX800 system was selected because production volumes had grown faster than anticipated. SMT line configurations included three SMT placement machines and the smaller CX600 system was becoming a constraint. The larger system supports the three placement machine configuration with no throughput issues.
Significant Quality Advantages
VP reflow soldering involves immersing PCBAs in a vapor blanket. This contributes to even heating and the vapor penetrates under every component. From a quality standpoint, this vapor blanket ensures perfect wetting and void-free, high quality solder joints. It also produces cleaner solder joints at lower temperatures (typically 240°C), eliminating the defect opportunities that can be caused by thermal shock during a convection reflow process that may reach 270°C.
Figure 2: A vapor phase “blanket” provides even heating and the vapor penetrates under every component.
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.
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Editor's Note: This article originally appeared in the January 2016 issue of SMT Magazine.