What Makes a Reflow Oven ‘Smart’ and Why Should You Care?

By Bjorn Dahle, President, KIC

There is much talk these days about the need for factories to transform themselves into smart factories that use intelligent machines to build intelligent products, all interconnected via the Internet of Things (IoT). That is easy to understand on a conceptual basis, but what does it really mean in practical terms? What makes a machine intelligent? Are there any criteria or metrics that can be used to evaluate the “smartness” of a factory? Perhaps more importantly, why should we care? What will intelligent machines buy us?

The ultimate objective for an electronic manufacturer is to manufacture the required quality in a sustainable way, to deliver products to the customer within the allotted timeframe, and to do so at a cost that allows the manufacturer to be profitable and the price to be competitive.

Investing in intelligent machines for its own sake is a waste of money; however, such investments are important in achieving the ultimate objective referenced above, particularly as we enter a new world characterized by:

• Mass customization resulting in frequent production changeovers of a large variety of products
• Falling prices on manufactured goods
• Heightened competition
• Sustainable manufacturing
• Demanding customers (think zero defects and full transparency and traceability)

It also should be pointed out that I am not suggesting burning down the old factory to build a new one. Most of the existing machines and infrastructure can be upgraded to smart technology.

There is a growing understanding in the electronics assembly industry that intelligent or smart machines demonstrate capabilities within a number of areas. Here I use the reflow oven as an example for illustrative purposes because that is where I have the most experience, but the principles are relevant for most, if not all, machines.

Process Transparency

The reflow oven is often thought of as a black box because you cannot readily see or visualize what happens to the PCB after it disappears inside the roaring oven tunnel, blasting hot air onto the unsuspecting boards for the next seven or eight minutes. Peter Drucker famously said: “You cannot manage what you cannot measure.” A half step in making the process transparent is to make the machine transparent. For a reflow oven, this means a continuous machine with monitoring capabilities to provide data on the selected oven recipe, actual zone temperatures and conveyor speed, time stamped board in – board out, board jam detection, and more. While such data is useful, it is insufficient. A smart oven will need process transparency.

A reflow oven’s main purpose is to create strong solder joints while adhering to the tolerances set by the relevant solder paste, component, and substrate vendors. To ensure that the oven produces each assembly in spec, the PCB thermal profile is measured. For process transparency, the profile and its relationship to the relevant process window must be measured continuously and in real-time. When profiling an oven on a periodic basis, the production is running blind and process transparency is nonexistent.

Periodic snapshots of the process (as in a manual profile) are similar to taking a picture with a still camera. You will have no dynamic information of the process, nor any visibility into the production in between the picture taking. Process transparency requires continuous and real-time measurements, as if a video camera was embedded in the oven and running nonstop. It is not enough to do good work some of time or even most of the time. Production now has to be right all the time, and data is required to prove it.

The process can be linked easily to the individual assembly by scanning each PCB bar code. We then have information about the process that each PCB experienced in the thermal process. Such information is of great value for traceability purposes, yield troubleshooting and much more. Significant cost savings are experienced through more production uptime as in the case of instant yield troubleshooting.

Flexibility

Market trends in the vast majority of industry segments indicate a strong movement towards the mass customization of products. That means shorter production runs and more frequent line changeovers. This is a challenge for many automatic machines because they tend to do a better job of cranking out identical products rather than constantly switching to different models or products. The reflow oven often is the bottleneck in the SMT line in terms of production changeover because it can take 30 minutes or longer for an oven to stabilize on a new temperature setting, especially if the new recipe is cooler than the previous. Careful production planning may yield significant improvements even without a smart oven. This includes starting the production day with the coolest recipe and progressing towards steadily hotter recipes throughout the day (assuming there is a production break within the 24-hour cycle to reset).

“A reflow oven’s main purpose is to create strong solder joints while adhering to the tolerances set by the relevant solder paste, component, and substrate vendors.”

A smart oven, however, will have software to search for the golden recipe that can process all the various assemblies in spec without any recipe change. The fastest oven changeover will be the one that does not require a changeover. The smart software literally will scan the billions of oven recipes and predict the resulting PCB profile for each. It then identifies the common recipe that can process all PCBs in spec. Whether such a golden recipe exists depends on the capability of the oven, the spread in thermal mass between the assemblies, and the variation in individual PCB process windows.

In many cases, the reflow oven is not capable of such a golden recipe. In that case, the software will search for new recipes by holding the zone temperatures constant and only vary the conveyor speed. While the oven may need 30 or more minutes to stabilize on new temperature settings, the conveyor speed resets virtually instantly.

In the worst cases, the reflow oven is not capable of processing all the assemblies using the same zone temperatures. The smart software will identify two or more recipes that can handle a wide range of assemblies using a method called grouping. You may think of this as a more scientific method to what happens in SMT factories every day where an engineer judges a new incoming PCB assembly to be processed using recipe A, B or C. There will be some downtime associated with oven changeover, but limited to the few groups that are set up as opposed to every new PCB.

For quicker NPI, ovens with intelligent databases will suggest the appropriate recipe without running a profile. The database uses collected information about the relationship between the oven thermal properties, PCB thermal mass, PCB process window, and the cause-effect dynamics between the oven recipe and the resulting PCB profile.

“Market trends in the vast majority of industry segments indicate a strong movement towards the mass customization of products. That means shorter production runs and more frequent line changeovers. This is a challenge for many automatic machines because they tend to do a better job of cranking out identical products rather than constantly switching to different models or products.”

Automation

Today’s SMT production line is highly automated but the level of automation soon will be reaching a completely new level. As humans, we all have different talents and we are capable of amazing things. What we are not, however, is consistent machinery that can be controlled every second of the day. Yet that is exactly what is required in order to manufacture the products right the first time, every time. Human labor is being freed up to focus on higher value work (and, one would hope, higher compensation).

“In the worst case, the oven is not capable of processing all the assemblies using the same zone temperatures. The smart software will identify two or more recipes that can handle a wide range of assemblies using a method called grouping.”

The smart oven will scan the incoming PCB, verify that the current recipe is correct, and verify that the oven is capable of processing that PCB in spec. If the current recipe is not suitable, the oven will load a previously optimized recipe before verifying that the oven is capable of processing the PCB in spec. Each and every PCB profile is measured automatically and compared to the relevant process window.

Automatic SPC charts measure process stability and give heads up information about signs of potential future trouble. Warnings on an out-of-control process and automatic conveyor stop for out-of-spec conditions will secure that each and every PCB is “made right the first time.”

All the process data for each PCB is recorded and stored for traceability purposes and can be retried at any time by scanning the bar code on the relevant PCB. The manufacturer will save labor cost and increase production uptime.

Interconnectivity

The reflow oven is only one cog in a bigger machine and it no longer can live in an isolated world. The dynamic real-time process data flows through the factory LAN for use in MES, ERP systems or other, as well as being accessible by other machines in the production line. The timeliness of the information enables better production execution as well as the entire shop floor management. For example, improved communication with material vendors is helpful for better inventory management as well as the occasional root cause analysis. Many EMS clients already can log into their own folder on the network, or in the cloud, to access all relevant information specific to the manufacturing of their products.

Learning and Adaptability

“Smart” is a relative term. There is an expectation in the industry today that machines will continuously learn and get smarter over time. Much work is yet to be done to make the machines more intelligent, but one can foresee a future where, for example, the AOI machine learns from the oven process monitor, as well as other processes on the line, in order to automatically correct upstream machines for improved performance. Already today, the aforementioned oven database keeps learning the relationship between oven thermal properties, the PCB process window, and the PCB thermal dynamics to build insight. Such insight will reduce or eliminate oven setup and changeover time. The automatic process monitoring system will learn about the absolute level of changes in the process, where in the oven they occur, and the rate with which the changes occur. The insight gained can help determine the timing of preventive maintenance, and even at which point the oven is no longer capable of executing the application, at which point it needs to be replaced, or the production shifted to another line.

Energy Efficiency

Most factories in our industry have programs for sustainability, reduced carbon footprint and other environmental considerations. The reflow oven is likely the most energy hungry of all the machines in the factory. Smart ovens will automatically optimize their electricity use for any given application. Case studies have consistently indicated an approximate 15 percent reduction potential in energy use due to the intelligent software’s ability to adjust and eliminate wasteful conditions.

“There is an expectation in the industry today that machines will continuously learn and get smarter over time. Much work is yet to be done to make the machines more intelligent.”

Retrofitable

Today’s SMT factories have a tremendous amount of sunk cost in their current previous generation equipment. Clearly, very few companies would want to sell off all existing equipment to acquire new smart technology in the new generation of capital equipment. Therefore, the smart technology needs to be retrofitable onto existing machines, preferably with a similar interface to make the user interface and interconnectivity easier to manage.

Smart Reflow Oven Benefits

• Process transparency leads to “making it right the first time” and having no “skeleton in the closet.” That translates into higher quality, elimination of rework and scrap, reduced risk and reduced cost. The consistent results are independent of model type, personnel, geographical location and PCB assembly.
• Flexibility leads to faster NPI and less downtime associated with production changeover, as well as shorter time to market. It allows a given factory to dramatically increase the number and variety of products being made while maintaining or increasing productivity.
• Automation secures consistent quality, reduced labor cost, higher productivity, elimination of running the production blind, process traceability and much more.
• Interconnectivity moves beyond sub-optimization of individual machines to managing the whole production line and the entire operation, including suppliers and customers. Near real-time data allows for productivity improvements all along the supply chain, and it allows customers access to comprehensive information about the production of their own products, along with peace of mind.
• Learning systems are not a one-time benefit but a continuous improvement cycle while the complexity and variability of the applications increase over time.
• Efficient energy use reduces cost and carbon footprint, and it moves the factory towards a sustainable operation.
• Overall a significant lower production cost.

Conclusion

Electronics manufacturing is changing rapidly, and will continue changing for the foreseeable future. Consumers drive the industry towards mass customization, faster time to market and lower cost. Smart machines are producing smart products and everything connected is gaining traction throughout numerous  industries. A key aspect of the new intelligent factories is not only to monitor what the machines do, but to focus on the outcome of those machine activities, providing real process monitoring.

The smart factories are more transparent for everybody, including their suppliers and customers, are highly automated and flexible, and are much more efficient in their use of energy. The factories are suitable for shorter production runs of vastly more diverse products, and they keep learning and getting smarter over time.

The bottom line is that manufacturers will consistently deliver the required quality, faster, and at lower cost. Now, that’s smart!

From the August 2016 issue of SMT Today magazine.