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Oven Profiling Aids Contract Wins
December 31, 1969 |Estimated reading time: 8 minutes
By James Hall, TRW and Brian O’Leary, KIC
In the highly competitive automotive electronics contract manufacturing market, full-service EMS providers constantly are challenged to improve processes and add traceability measures to their assembly processes. This article documents how streamlining and upgrading their thermal profiling capabilities allowed one company to address productivity, quality, and client relations in the automotive electronics sector.
For companies developing and manufacturing in the automotive electronics space, top-notch quality and on-time delivery are always expected. But even this is not enough to secure success. Intense competition for all new contracts places immense pressure on prices.
New technologies and more effective ways of doing business enable high-reliability manufacturers to remain profitable while exceeding client expectations. The thermal process offers an opportunity for improvement on three fronts: productivity, quality, and client relations.
Productivity
To set up the thermal process for a new board design, numerous iterations of profiling and oven recipe adjustments typically are required. This is costly in terms of delayed production start and labor cost. Additionally, a typical golden board with thermocouples used for profiling typically lasts no more than 10?20 runs. At one EMS facility, estimated annual cost of golden boards reached approximately $70,000, including replacement thermocouples, and repair and replacement of damaged profilers. Additional productivity issues relate to frequent and slow line changeovers, suboptimal recipe settings with a slow conveyor speed, and excessive downtime due to manual periodic profiling.
Quality
Even with low ppm defect levels for thermal processing at automotive electronics manufacturing facilities, the old fashioned method of periodic “snap-shot” profiling is inadequate and risky. Essentially, production is running blind because periodic checks and various oven alarms are incapable or unreliable in detecting problems that can occur during production. The manual profiling method itself can be unreliable. Whenever a new golden board is made up, slight variations in thermocouple application will impact the measured profile. Once a profile is attained, the manual interpretation of its suitability is subjective, leading different technicians to come to different conclusions. An out-of-spec process may incorrectly be deemed acceptable and production could continue. No-clean solder pastes make flux management more difficult because of the added amount of solvent and flux to the solder paste. Fine-tuning the reflow oven exhaust flow is difficult, and it increases the risk of condensing flux back onto production boards.
The introduction of lead-free alloys also elevated the need to improve thermal management for heat-sensitive surface mount components subjected to higher soaks and peak flow temperatures. Smaller process windows forced tighter thermal controls. Not all component suppliers are ready available to make lead-free conversion of safety-critical components in the timeframe that the Waste from Electronic and Electrical Equipment (WEEE) or End of Live Vehicle (ELV) clauses put into effect. This imbalance between higher operating temperatures and heat-/moisture-sensitive component packages forces automotive electronics assemblers to establish better strategies for managing and protecting heat-sensitive components within a global profile.
Figure 1. Using thermal profilers for process development and management can raise productivity and maintain quality at high-reliability EMS facilities.
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Client Relations
Clients’ auditors frequently challenge EMS providers to improve services, including documentation. They also demand annual cost reductions. Automating routines, such as paperwork, reduces costs for EMS suppliers.
One solution is a two-tier approach, with process development work and process monitoring comprising the system.
Process Development. A thermal profiler with recipe optimization software can mathematically calculate each profile’s fit to its process specification. A process static analysis signals either a process perfectly centered inside the available process window, reflow teetering at the very edge of its process window, or out-of-spec production. While setting up a reflow oven for a new board design, an appropriate thermal profiling tool will, after a profile run, calculate the profiles and their fit to the process specifications for all the available oven set-up metrics.
A modern reflow oven may have billions of different set-up combinations. An optimum profile may be defined by the engineer as the one most centered in the process window, offering the fastest throughput/conveyor speed, or a combination of the two goals.
Process Monitoring. Other thermal process monitoring options embed sensors permanently in the reflow oven process tunnel. This solution automatically calculates the profile and its fit to specifications for each processed board in the oven without any labor input or any interruption to the production. It can set off alarms on any out-of-spec situation, and may shut down the feed conveyor to prevent any PCBs from entering the oven until the situation has been corrected. The system also may monitor the process control (Cpk) level for each processed board, again alarming on an out-of-control process that could, unnoticed and unchecked, lead to an out-of-spec situation.
Results
When upgrading existing reflow ovens at an automotive safety electronics facility with new process development and process control tools, we experienced a host of expected improvements, as well as some unexpected ones. Because of an initial high quality level, the most significant financial rewards came mostly as a result of improved productivity.
Periodic profiles ? which had been run for each product changeover, as well as on a weekly basis, and after each preventive maintenance workup ? were eliminated. It would take 15 minutes to run each profile. The financial benefits relate to labor cost savings, production uptime, and the annual $70,000 spent on profiler repair/replacement and golden boards.
We also were able to identify groups of board types that could be run under the same recipe, resulting in dramatic production changeover time improvements. The reflow oven needs anywhere from 10 to 35 minutes to stabilize on a new temperature, especially if the new recipe calls for cooler temperatures. If the recipe does not change, or if the new recipe only calls for a different conveyor speed and no temperature changes, the oven changeover is measured in seconds. Finding a common recipe for a group of different products, or being able to identify acceptable recipes where only the belt speed has changed, significantly impacts production uptime.
Figure 2. Constant process monitoring within the reflow oven tracks a profile from ramp up through peak and dwell, and into cool-down stages.
In the past, an operator would need numerous profile passes to set up the oven for a new board design. Due to optimization software, this takes a fraction of the original time, using few profile passes. Additionally, conveyor speed was increased from 34 inches/min. to 45?47 inches/min. for this facility.
The flux vehicle for no-clean pastes has been modified to reduce the amount of solvent loss in the printing process to provide longer stencil life. This increasingly burdens the oven, which needs to remove the solvent and flux that have been added to the paste. Different exhausting configurations in the reflow oven would have a critical impact on how well we were able to channel the evaporated flux away from the process chamber. The challenge was that the different exhaust flows also affected the profiles. The thermal process monitoring system allowed us to tune the exhaust flow for optimal flux removal while running an in-spec profile. This reduces the risk of flux condensation on processed boards, not possible with the previously used method of manual profiling.
There is a host of possible reasons for line-stopping production problems, including a clogged stencil, misplaced or faulty components, component lead co-planarity issues, out-of-spec thermal process, and more. Ironically, the thermal process tends not to be the culprit in these situations, but because we were essentially running “blind” as far as the reflow oven is concerned, it often forced us to check the profile first when troubleshooting the line problem. Checking the profile typically would take 15 minutes, and often resulted in a confirmation that the thermal process was acceptable. The continuous monitoring system would let us know whether the problem was related to the profile or not, saving 15 expensive downtime minutes.
The same goes for field returns. It is always important to identify the original cause of a field failure to prevent future occurrences.
Clients take a keen interest in how production lines are run. After all, their name will be on the final product. Automotive quality auditors often are pushing their suppliers for continuous quality improvements, and to keep up with available technology.
Clients support continuous thermal process monitoring because it, in real time, alarms on an out-of-spec situation, something that might escape the eye of an operator. It also enables them to trace field failures back to the original process.
Conclusion
High-reliability manufacturers need to consistently and cost-effectively produce top-quality products in all production lines in all factories globally. Incorporating process control limits for reflow in the EMS provider’s control plans effectively means that all products will be in spec and no products will be processed beyond the edge of the acceptable thermal process window, as defined by the user. It improves consistency around the world, regardless of the various oven types used on various lines in different facilities.
Figure 3. An operator tracks the profile on the reflow oven.
More high-reliability manufacturers are implementing lead-free in assemblies, for various reasons. Lead-free offers environmentally friendly products and encourages recycling. The problem with lead-free electronics is that the thermal process window shrinks significantly. Reflow optimization software enables us to customize thermal specification for individual components (heat-sensitive) within a global array. This feature enables users to meet the needs for all components on the same substrate. In addition, the process monitoring system acts as a watchdog that will shut down the feed conveyor should the process drift outside of the specified limits. SMT
James Hall, process engineer, may be contacted at TRW, 2150 Cranebrook Dr., Auburn, N.Y. 13021; (315) 355-3311 ext. 411; Fax: (315) 255-1995; james.hall@trw.com. Brian O’Leary, North American sales manager, may be contacted at KIC, 15950 Bernardo Center Drive, #E, San Diego, Calif. 92127; (858) 673-6050; Fax: (858) 673-0085; boleary@kicmail.com; www.kicthermal.com.