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Inside Reflow
December 31, 1969 |Estimated reading time: 17 minutes
By Gail Flower, Over time, the basic equipment for mass board assembly changes, adapting to various updated practices. In soldering, for instance, some companies expand their product line with wave soldering machines — such as Qualitek’s JT NSM — and reflow ovens, adding to their traditional offering of solder pastes. Manncorp Inc.’s Henry Mann mentions that his firm now offers reflow and wave solder machines with smaller footprints, and energy-efficient wave systems using less solder. Martin Ziehbrunner, CEO of Essemtec AG, comments that the market for reflow ovens remains influenced by the fact that most Europe-based companies invested in 2006 for the RoHS conversion. Celestica, an EMS provider dealing with product complexities such as stacked packages, now is considering different technologies, including vapor phase reflow, according to Peter Tomaiuolo, director, corporate technology.
Reflow soldering ovens remain the dominant form of soldering equipment, with the market for reflow ovens experiencing a significant increase in compound annual growth rate (CAGR) during the run-up to July 2006 RoHS compliance, according to Bob Klenke of ITM Consulting. This was followed by a period of moderate demand; resulting in an overall 2001–2011 market forecast that, when adjusted for fluctuations in demand, shows a positive growth trend. Klenke sees a growing market in the Asia-Pacific region, specifically China. “Asia accounts for about 70% of worldwide sales of reflow equipment, with the balance equally divided between Europe and North America,” he estimates.
Robustness seems to be what every user requires of reflow. Producing quality solder joints with low solder and material defect levels over the range of paste print and placement variation from upstream assembly processes is important. Much of this robustness is determined by the chosen profile’s shape. Another measure is how repeatable the heating and cooling process remains over time and under varying loads. In the best scenario, the ability to create a wide range of time/temperature profiles for a variety of products with acceptable thermal uniformity across each product is the goal.
Some systems offer cooling controls, either through regulation of gas temperature or flow rates. However, the level of control is never as great as in the heating zones. Most ovens have a relatively low number of independent cooling zones, adds Phil Zarrow of ITM Consulting. The net result is that compromises must sometimes be made in optimizing cooling slopes, which in turn affects the rate of cooling for critical materials on specific products.
User Survey Responses
How have your reflow needs changed?Most respondents mention the need to comply with RoHS lead-free reflow soldering as a motivation to purchase machines with better controls. Most use larger ovens with additional zones to meet ramp peak times. In the group, 2.5% use wave soldering, 92.5% convection, and 5.0% vapor phase.
“The move to RoHS-compliant processing has forced a change as to how we specify reflow ovens for our reflow processes,” says Thomas Chapo, senior engineer at Applied Technical Services Corporation (ATS). “Early on we realized that RoHS was coming. When we needed to upgrade our reflow ovens, we made sure that we were ready and compliant. As the company grew, we made being RoHS-certified-capable in our reflow process a key metric.”
Diego Stelling, process engineer at Foxconn CMMSG in Brazil, says that his company started up a new operation in Jundiai-SP and, therefore, purchased three Vitronics-Soltec XPM2 for lead-free use.
Others note the use of nitrogen in new equipment. Sylvain Kaufmann, production technician at CERN/European laboratory for particle physics, talks about the need for accuracy &ndah; it has increased with the growth of component size and sensitivity variety. “Flexibility became a major factor.”
Piotr Panufnik, engineer at TRW Automotive, notices more changes. Since lead-free practices have become more realistic production challenges, the oven zones have increased to 10 or 12; some manufacturers are starting to add nitrogen; and the continuous process monitoring systems like the KIC 24/7 or ECD M.O.L.E. for PCB profiling are more important to those building boards. Helmuts Grabis, chief engineer at Hansa Electronics, adds that his firm now uses nitrogen in their convection ovens.
It’s not just lead-free solder pastes that require special attention in the reflow process. Johan Schmidt, process development engineer at Enics Sweden AB, sees a need to double the capacity for wave and selective soldering equipment due to lead-free demands from ODMs. He says convection machines have more sections to optimize the reflow profile. However, smaller-pitch components and BGAs have influenced his choice of equipment.
What is a robust reflow process?Most respondents declare that customers require zero defects and they must meet those needs while running machine in a 24/7 operation. The uptime repeatability of the machine is how robustness is measured.
Figure 1. Manncorp offers a range of systems for EMS companies.
“We are an EMS provider, so we must have processes that are flexible and robust,” says Chapo of ATS. “For that reason, all of our RoHS-certified-capable ovens are at least 10-zone machines. The extra zones give us a lot of cushion in most instances for process robustness and throughput capability. Being RoHS-certified by the equipment manufacturer means that we do not have to worry about the toll that the higher processing temperatures will take on the machines, as with trying to run RoHS products on machines designed for tin/lead processing.”
Baharudin Saad, SMT process engineer at Flextronics Technology Saha Alam, Malaysia, determines robustness by the consistency of equipment reflow profiling allowing for small tolerance: peak temperature 235°-245°C; time above 217°C between 45 and 55 sec.; preheat 150°-220°C, time 80-100 sec.; rising slope 30°-150°C; time between 190° and 217°C, 15–30 sec., specifically.
Temperature and ΔT during process steps (preheat, soak, peak, etc.) and repeatability from week to week are important to Kaufmann at CERN. He measures these using generic boards pre-wired with thermocouples connected to a thermal profiler.
Many reflow users mention testing procedures performed afterwards to see if board reflow made all joint connections. In-line automated optical inspection (AOI) and automated X-ray inspection (AXI), along with temperature profiling, are top choices.
How do you decide on equipment?For resources that help engineers decide which products will meet their needs, many turn to magazine articles, research papers, and company reps. They do comparisons of features and price. They talk to peers and co-workers to see what types of experiences each has had in the past. They look at support, services, downtime, lead-free compatibility, upgradeability, specifications like machine dimensions, and software. Larger groups rely on global plans, such as Flextronics’ FlexGlobal committee.
Figure 2. Ronny Horn, Ph.D., and Heike Schlessmann demonstrate Seho reflow ovens at a tradeshow.
CERN’s equipment is preselected with user feedback. Technical articles assist in the process. Preselection relates to their needs with batch size, PCB assembly (PCBA) variety, and, of course, price as determinates. Kaufmann visits exhibitions and conferences, like Productronica, as well.
How long does reflow equipment last?This question left those surveyed perplexed. They expect all machines to last beyond five years and hope that reflow equipment would last beyond 10 years. One engineer does have reflow equipment running more than 10 years. Generally speaking, equipment usefulness is related to the constancy of its use, but, most of the time, financial ROI rather than machine decline helps an engineer decide when equipment can be replaced. In one case, a respondent leases reflow equipment on a three-year contract.
How much of your budget goes to reflow?Answers ranged from 5 to 15% of an annual budget. However, once the oven is purchased, only operating and maintenance costs are dealt with yearly, and these costs typically are not high. “Reflow machines are not cheap, but relatively speaking, they do not stand out as being a higher priced machine,” says Chapo at ATS. “Like most of the equipment in an SMT line, reflow ovens are expected to perform for years without needing to be replaced. The cost of maintenance is low on an annual basis. We have not seen the need to run nitrogen, so that helps keep our processing costs down.”
What problems are attributed to reflow?brProblems attributed to reflow are ubiquitous: solder opens/shorts, tombstoning, poor wetting, solder balling, bridging, solder insufficiency, half-baked joints, uneven heating, uneven transport, temperature consistency, popcorning, dewetting, cold solder joints, BGA issues, and intermittent test failures. BGAs are the most problematic parts when mounted on boards, and an uncontrolled reflow process could cost lots of engineering and troubleshooting money.
Bad profiles can create defects, notes Stelling, Foxconn. Defects are preventable with proper preventive maintenance to equipment, adds Kaufmann, CERN.
“We have one machine that has provided us with the bulk of issues,” says Chapo of ATS. “All of our other machines are from another manufacturer and are the same basic model. They do not create any issues. The problematic system was one of the first attempts at a RoHS-compliant machine; there are some issues with the conveyor chain and keeping it lubricated well enough for the higher process temperature it sees. This mostly is because of the design approach for that machine.”
How do you control the reflow process? Amol Kane, Harvard Custom Manufacturing, uses SPC and oven monitoring to control the process. Schmidt of Enics Sweden AB continuously checks processed boards with AOI and AXI. If the results start to deviate, an alarm alerts service engineers. Once a week the temperature profile is checked with a mole. Richard Anderson, senior principal engineer, Tyco-MA-COM, uses thermal profilers for monitoring. Michael McCallion, global lean and best practice director of CTS EMS, uses moles and does constant monitoring with CPK measurements. Saad of Flextronics Malaysia monitors profiles daily.
How many PCBs do you reflow per month?The answer to this question ranged from 500 to 400,000, depending on the type of company responding. No matter the amount of boards processed per month, many comments regarding reflow were similar.
Supplier Survey Responses
The equipment suppliers mentioned many types of reflow, from convection with air or nitrogen atmosphere as the dominant form to infrared (IR) for curing applications, vapor phase, laser, or some combination.
What range of equipment do you offer?Denis Barbini of Vitronics-Soltec says, “we deliver an extensive range of reflow equipment. From mid-range reflow machines like the MR 933 to high-end ovens like myReflow and the XPM3. Depending on the user requirements for throughput, the models can be delivered in different configurations and lengths.”
Mark Peo, president of Heller Industries says, “we offer the compete range from 6 top- and bottom-heated zone system for low-/medium-volume production up to 13 top- and bottom-heated zone systems that can solder a motherboard every 12 sec.” Nitrogen and air operations are available, as well as dual-rail reflow for high throughput. The flux elimination system keeps the oven tunnel clean, lowering maintenance time.
BTU International offers convection reflow ovens for board and semiconductor assembly. Their Pyramax family of reflow ovens is available in 6-, 7-, 8-, and 10-zone configurations. Kristen Mattson of BTU talks about the ability to have closed-loop convection for precise control of heating/cooling. Constant heat transfer delivers process control flexibility and ensures repeatability from side to side and line to line.
Manncorp’s offerings include high-throughput lead-free systems with 5, 6, 7, 8, 10, and 12 heating zones independently controlled. “Recently introduced is our five-zone small-footprint lead-free system with on-board computer and profiling,” says Henry Mann. The company also has benchtop low-volume batch-type ovens.
SEHO offers PowerReflow convection ovens for medium to large production series. Its MaxiReflow can have an arrangement of up to 12 zones. Both are configurable for nitrogen, and equipped with transport adjusted via an under-rail cable, keeping the chain-guiding system taut and in parallel.
Tamura H.A. Machinery, Inc. offers convection reflow with air or nitrogen. R&D Technical Services offers a full line of convection reflows from low-volume batch types to high-volume in-line equipment. Qualitek offers JT VS/NS 800-1000 series reflow ovens. The 800 series has eight top and bottom heating zones at a 123" length. The 1000 series has 10 top and bottom heating zones at 154". The VS series has a built-in KIC auto focus with SPC charting.
Essemtec offers a tabletop convection reflow oven for prototypes and stand-alone reflow ovens for small batches. They also have in-line reflow ovens for medium volumes.
ATV Technologie GmbH sells solder reflow ovens with rapid thermal annealing capability under controlled atmosphere/vacuum pressure, designed mainly for R&D process development and low- to high-volume production.
Figure 3. PCBAs enter an Essemtec oven.
Two companies that responded to our survey produce and sell vapor phase equipment. R&D Technical Services, for instance, has a full range of vapor phase reflow ovens from low-volume batch to high-volume in-line equipment. IBL produces a complete line of vapor phase equipment from tabletop lab ovens to high-throughput in-line equipment.
Figure 4. Qualitek integrates KIC thermal profiling on certain machines.
Two others worked with reflow providers closely; however, their main business is to provide profiling and control systems. Bjorn Dahl of KIC says that his company “offers standard manual profilers, process optimization software, automatic profilers, and continuous process monitoring and traceability systems.” Grant Peterson of ECD states that his company has “thermal monitoring tools for process measurement at the PCB level, machine maintenance verification, continuous zone monitoring, and SPC on key process indicators.”
How does lead-free affect reflow?Most suppliers agree that the lead-free initiative continue to affect all areas of manufacturing worldwide, none moreso than the reflow process. “The higher melting temperatures and soak duration of lead-free solder formulas require a change in the way people manage the reflow process,” says Mattson at BTU. Soak times, peak temperatures, time above liquidus (TAL), and ramp rates all require tight control within a shrinking process window. Cooling requirements, exit board temperatures, and flux management should be considered carefully. “The reflow process is affected by higher melting temperatures of solder pastes,” adds Barbini of Vitronics-Soltec. The typical melting temperature for lead-free is 230°-235°C. Peter Kim of Tamura adds that lead-free requires longer tact time, higher temperature, and precise ΔT control. Debbie Liguori of Qualitek says lead-free solders require higher melting temperatures; however, paste manufacturers will provide recommended reflow profiles for their formulations. The melting temperature of the solder, heat resistance of components, and PCB characteristics determine the actual reflow profile, she adds.
Since lead-free solder doesn’t wet as fast nor spread out like tin/lead solder, it reflows where it was printed. This can force a change in the stencil design with the aperture opening, according to Qualitek. Board material is another consideration beyond reflow. Dahl, KIC, adds that hiking the process temperature 30°-40°C raises factories’ energy usage. Also, due to component and substrate tolerances, the lead-free process window is tighter.
If the board enters the AOI machine too hot, this can play havoc and tend to give false readings, according to Heller’s Peo. “Additionally, solder paste manufacturers have found that a 3°-4°C/sec. cooling rate may be needed to provide good grain structure and shiny joints. Many existing machines do not have such a fast cooling capability, so the cooling aspect of the profile should also be confirmed for both the reflow oven and post-reflow AOI,” says Peo.
How has the market changed?“The market has changed in two aspects: the lead-free conversion and customer requirements for tighter process control,” says Barbini. “The conversation about having a lead-free oven has become a moot point. It is expected that all ovens will be lead-free ready,” adds Mattson, BTU. There are “higher requirements regarding even temperature distribution, accuracy, control, uniformity, cost of ownership (COO), and reduced power consumption,” says Steve Pollock of Essemtec. PowerReflow was designed for flexible applications, whether it’s single- or double-sided reflow, or high-temperature processing, or curing adhesives and underfills, adds John Welling, VP, Seho.
Vapor phase once again has become an option. “The reflow market has improved for vapor phase reflow over the last five years due to lead-free processes and more difficult/sophisticated products,” says David Suihkonen of R&D Technical Services. A large BGA or an expensive stacked 3D chip with logic and memory will respond well to the gentle reflow of vapor phase, says IBL’s Andreas Thumm.
Mattson notes that the geography of the market has shifted. Today, Europe and the Americas hold a less influential position in global marketplaces as compared to Asia. “Five years ago, some emerging Asian facilities still accepted equipment from decision makers at American or European facilities. Now many decision makers are at the factories in Asia,” Mattson explains. “There are fewer lines of equipment in the U.S.,” says Liguori of Qualitek, “combined with the import of Chinese-made machines, it makes the market very competitive for the reflow business.” This depresses prices while upholding high quality standards. “Price competition has increased significantly, partly as a result of numerous reflow oven and wave solder manufacturers from China and Taiwan entering the market,” adds Dahl of KIC.
Kim at Tamura mentions that the addition of nitrogen gas and lead-free adjustments is a real change in the market. He explains that 80% of the Asian market is already using nitrogen.
Peo at Heller says, “We see customers trending away from ultra-large ovens to a more value-oriented/COO model.” Though performance is paramount in producing high yields, a low COO combined with good ROI is the primary focus.
Conclusion
Curious, we asked each reflow equipment supplier about system limitations. Most did not partake, or simply said, “None.” Others noted that their equipment targets a certain user segment.
Mann of Manncorp says their equipment is designed for mid-volume PCB assemblers, and Pollock explains that Essemtec’s machines were not made for high-mass manufacturing. Peo works with key customers in Japan, China, Korea, Europe, and the U.S. to tailor ovens for market needs; however, there’s always room for improvement. With tongue-in-cheek humor, Dahl of KIC assures us that his profilers “don’t do windows.” Ligouri at Qualitek quips, “We are still working on a good cookie recipe, or maybe we’ll do brownies.”
Gail Flower, editor-in-chief, SMT, may be contacted at (603) 891-9395; gailf@pennwell.com. For more reflow information, including expanded content from SMT’s surveys, visit smtonline.com.
Achieving Controlled Reflow
High and uniform heat transfer is the foundation capability of any reflow oven. It’s almost always demonstrated by establishing a profile on a thermally complex product and measuring temperature uniformity across the entire board, especially at peak temperature.
Another criterion is the capability of a reflow oven to generate the desired profile shape on various boards. Although most process engineers would prefer to use a straight ramp profile, sometimes a form of ramp-soak-spike profile improves thermal uniformity of complex products, or a shoulder profile reduces defects, especially tombstoning. Accurate control of profile shape is achieved within an oven through zone definition. This is the ability to operate adjacent heating and, potentially, cooling zones stably under load and at significantly different temperature set points. All ovens exhibit some limit to zone definition, above which heat from the higher-set-point zone spills into the adjacent zone, causing it to rise above its set point, effectively creating an out-of-control condition. Ovens that can maintain a high level of zone definition offer more capability to generate a wider range of profile shapes.
Repeatability is probably the most important criterion for a reflow system. This is the capability to reproduce the exact time/temperature profile on a product at any time and under any loading conditions. Once a recipe (conveyor speed, zone set points, etc.) is established that produces a profile that meets all reflow specifications, every board should see that exact profile, regardless of external conditions. Fluctuations due to loading are the biggest concern, since problems in upstream printing and placement occur, frequently causing interruptions in the flow of product into any oven.
Controllable cooling is one of the most overlooked attributes in a reflow oven. Numerous factors in the increasing complexity of electronic assemblies are creating the need to shape the cooling section of the profile in the same manner as heating sections. The final structure of the solder joints (shape, grain structure, etc.) is formed within the cooling section. Other concerns include protection of critical components and materials, preventing warpage in complex IC packages and high-density substrates with microvias.
Most top-of-line reflow systems available today provide much of this functionality, but, as always, users should define all of the specific requirements of their unique products before evaluating a specific oven model.
Jim Hall, principal consultant, ITM Consulting, may be contacted at (603) 868-1754; jim_hall@itmconsulting.com.