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Cleaning Up The Process
December 31, 1969 |Estimated reading time: 7 minutes
Two companies worked together to develop a process to control flux flow, eliminating buildup and reducing maintenance.
By Michael L. Martel
As a contract manufacturer, Primetech is exposed to many types of assembly needs. Many of the company's manufacturing lines are equipped with nitrogen reflow soldering systems*. For products that require a water-wash chemistry, other lines are equipped with systems that operate in air only**.
For products where the primary chemistry used in the solder pastes and fluxes is no-clean, the systems are operated with nitrogen inerting to deliver optimum results with typically low-solids flux formulations. The difference in quality between products processed in nitrogen or in air is considerable when using no-clean products or in applications where the process is particularly vulnerable to the formation of oxides. Nitrogen widens the process window by improving the wettability of surface mount component leads and improves overall joint integrity. Nitrogen processing for higher technology assemblies such as ball grid arrays (BGA) and microBGAs including 0.016" fine-pitch SMT is also used. The company dedicates the nitrogen process to those types of applications, maintaining atmospheric purity levels down to around 10 to 15 ppm of O2.
Flux Buildup a Sticky ProblemLike all closed forced-convection reflow systems, flux residue management has been a perennial problem. Forced-convection SMT reflow systems operating with inert atmospheres recirculate process gases, often concentrating flux vapors within the process area. Although these vapors are eventually trapped and removed via filters and condensers as well as exhausted, some of these residues will find their way into the cooling module, where they condense on components and form sticky, corrosive buildups. Such buildups, without maintenance or servicing, can become massive, and dramatically reduce the cooling performance and efficiency of the cooling module itself. Worse, these concretions eventually ooze and drip condensed fluxes onto exiting product and the transport system itself, contaminating the product. The only solution to the problem has been to shut down the line, dismantle the reflow oven's cooling module, and clean it in a batch cleaning unit with scrubbing and manual cleaning techniques. The process is dirty and costly in terms of machine downtime, and ties up personnel and batch cleaning equipment.
Some oven manufacturers have developed a method of coping with these contaminants by redirecting the process gasses to an outside filtering system. This eliminates system downtime because the oven does not need to be shut down to service the filtering system. The drawback of these filtering systems is that they still require removal and cleaning. They also add to the machine's power consumption and can add to its nitrogen consumption as well.
This sort of buildup is not a problem with reflow systems operating strictly in air. The air flow in the machine allows the flux fumes to be exhausted rapidly enough from the process area so they do not have the opportunity to concentrate and condense on cooler surfaces and cause maintenance problems.
In some cases, companies have purchased an identical extra cooling module, removing the dirty one and swapping in the extra one while the other is cleaned. While this rotational procedure saves time, it requires the purchase/inventory of spare cooling module parts and scheduled downtime (including cleaning), the frequency of which is dictated by production volume. The higher the production volume, the more frequent the cleaning procedure. At one point, the company was cleaning its equipment once every three months. With greater throughput demands, that requirement was rapidly increasing to once every 30 days.
With high-volume production, downtime for any line, regardless of the reason, can have a serious impact on production. "We are constantly looking at ways to reduce downtime," said John Gileta, manager of process engineering. "Even though our maintenance was well planned, we still would have to stop the line to perform the laborious task of removing flux residues from the cooling zone of a reflow oven. This time could have been better spent in other areas."
As production volumes were constantly increasing, the company continuously worked on the optimization of processes and equipment, which brought the product changeover time down to 15 to 20 minutes per line. Unfortunately, frequent maintenance because of flux accumulation began to eat away at these gains.
Vitronics Soltec engineering personnel recognized the severity of this industry-wide problem, and had been working on a solution for some time. The company's engineers went back and reviewed the basic design concepts for its process, the patented individual zone intakes and exhausts, and saw the opportunity to apply a theory of flow control that had been developed. The engineers believed that by changing the balance of atmospheric pressure and flow direction within the system, the recirculation of the process gases and low oxygen ppm levels could be managed without requiring an increased flow rate. This would prevent process gases from entering the cooling zone, virtually eliminating the need to dismantle the cooling module for cleaning (Figure 1).
Figure 1. Flow scheme of flux flow control technology.
An Eight-hour RetrofitA prototype system was built at the supplier's headquarters and research facility in Stratham, N.H. After some slight modi-fications, test results showed that the system would perform as expected, but high volumes of flux fumes in a real manufacturing environment would prove whether or not the new process was viable.
Primetech was chosen because of their close proximity to the facility. They also had a demanding high-volume environment that would prove to be an appropriate "trial by fire" for the new flux flow control technology. They would also stand to benefit greatly if the process worked. It was determined that it was not necessary to remove equipment from the company and plans for the in-field retrofit began.
The engineers practiced the retrofit procedure and found that one machine could be changed over in an eight-hour shift. Primetech gave them the go-ahead, and the team of technicians completed the retrofit on two units in the promised time and put the machines back on-line. They then waited to see the results (Figure 2).
Figure 2. The flux flow control technology debuted on Primetech's factory floor.
Results Exceed ExpectationsThe modified ovens maintained a consistently low ppm oxygen inert processing environment without accelerating gas usage. A positive inert gas pressure kept contaminants out of the cooling module. The result is a clean, efficient inert processing, with no flux-encrusted condensing modules or filters to clean. Boards and assemblies remain free of contaminants, and product/process cooling modules do not become dirty and require dismantling for cleaning or replacement. Field tests have shown that such service is up to eight times less frequent than with systems not equipped with this feature. Eliminating the need for flux filters further reduces maintenance requirements.
At the time this article was written, the retrofit flux flow control has been in place for more than four months. "Our production volume has increased three-fold," Gileta said. To date, there has been no noticeable accumulation of flux in the cooling module, so maintenance time has not been scheduled. Additionally, there has been an unforeseen benefit to the new technology: increased stability for the inert "blanketing" at lower O2 ppms within the machine, with the use of only a slightly higher flow rate of N2.
More Robust Inert Processing"Typically, with inert reflow processing, the reflow machine is sensitive to atmospheric changes within the processing facility. Opening a door or window, or the presence of ceiling fans any type of atmospheric change or turbulence within the room can have a temporary and destabilizing effect on the nitrogen blanketing within the system, no matter how tightly closed that system may seem to be," said Russell Shaw, senior project leader at Vitronics Soltec. "Our flux flow control does not require any additional nitrogen usage to maintain desired ppm levels. A slightly higher flow rate enhances the stability of the machine, enabling greater consistency to the nitrogen blanketing, and thereby making the process more robust."
"The flow rate we were using around 900 square cubic feet per hour (SCFH) was actually rather efficient, not excessive at all," Gileta said. "But a slight increase and by no means a significant increase at all has given us a more stable and robust inert process. Also, we are saving on downtime and maintenance, since all of the dismantling and cleaning process appears to have been eliminated. We also have no filters or condensers to service."
More Effective Cooling"Another improvement that we noticed that was also unexpected was an increase in the efficiency of our cooling module," Gileta explained. "This was not so much due to the lack of flux buildup, because our maintenance program did not allow that much buildup to take place, but we believe that it is due more to the change in direction of gas flow. Instead of hot process gases creeping into the cooling area and reducing the efficacy of the cooling module, the gas moves the other way. Its flow does not affect the heating zone at all, but we have measured consistently lower temperatures in our cooling zone, and that's certainly good news."
- Vitronics Soltec M-Series.
** U-Series.
MICHAEL L. MARTEL may be contacted at Martel Marketing Communications Inc., 16 Birchwood Drive, Bristol, RI 02809; (401) 254-1473; Fax: (401) 253-7353; E-mail: mmc@loa.com.