Reflow in Today’s High-Mix / High-Volume Production Environment
By Bjorn Dahle, President, KIC
Many printed-circuit-board production lines working in surface-mount-technology (SMT) markets in the US and Europe can be described as high-mix, low-to-medium volume lines. Many of these lines experience more downtime than production time, creating a serious challenge: how to increase the volume while also increasing the efficiency.
The downtime is mainly caused by extended machine setup and changeover times due to the high mix of components on SMT boards.
One critical machine that can contribute to a great deal of the downtime and/or the potential success of a SMT production line is the reflow oven. Reflow oven setup time can be quite long for a new PCB assembly, often the slowest changeover for any machine in the production line. Long times to prepare a reflow oven can impact delivery times and profit. While many technologies have been developed for lower-mix, high-volume production lines, less attention has been paid to the production needs of high-mix PCB assembly; saving reflow oven setup time can boost the efficiency of any high-mix, lower volume PCB production line.
Manufacturers and suppliers of solder paste, substrate materials, and electronic components provide tolerance or process windows for the safe use of their products. A reliable manufacturing practice involves the attachment of thermocouples (TCs) to each unique PCB part number to ensure that each manufactured PCB remains in-specification (in-spec) for the process and oven limits determined optimum for that PCB. An in-spec profile is determined by measuring a time-versus-temperature profile that considers interfaces between multiple process windows:
- A solder paste specification or specifications for other materials such as underfill
- Tolerances for different electronic components
- Substrate tolerances and processing requirements
- Design efforts to reduce the process window by creating an effective thermal layout, such as positioning components with larger heatsinks next to smaller SMT components
Consideration of a PCB substrate and its place in a reflow oven process includes the choice of PCB glass transition temperature (Tg) and decomposition temperature (Td) and other factors, such as copper weights and the number of circuit layers. Another factor is the type of surface protectant used with a selected PCB material, since a PCB assembly may require multiple reflows and cleaning.
Jose del Valle, Chief Operating Officer/Vice-President of contract manufacturer NuVal EMS (Tustin, California), believes that it is not sufficient to process MOST components within specification. Rather, ALL components must be held within the relevant process window. This is made more difficult by the large numbers of components mounted on typical PCBs. The relative positions of the components and their thermal masses and thermal process requirements can also determine the thermal characteristics of a particular PCB.
Best Manufacturing Practices
For companies such as NuVal EMS attempting to apply best manufacturing practices to the solder reflow process, such practices as thermal profiling, traceability, process control, and inspection are among the tools that can help improve quality and productivity while reducing manufacturing costs. Commercial oven setup software tools such as KIC Navigator and KIC Auto-Focus from KIC can dramatically reduce reflow oven setup time. KIC Navigator can create optimum oven profiles based on a single existing profile.
KIC Auto-Focus works as a form of KIC Navigator with a data base, searching for the best oven profile for a particular PCB. With this software, a technician simply supplies the length, width, and weight of the PCB, and the system recommends an optimal reflow oven recipe within a few seconds, without running a profile. This software-based approach is an improvement over guessing which oven profile to use for a particular board, and a verification profile can be run afterwards to ensure that the oven parameters are the best for that PCB.
Oven Setup Software
Reflow oven setup software evaluates the large amounts of data represented by the many different thermal profiles for the many components mounted on a PCB to be manufactured. It can also determine the effects of oven conveyer speed in its search for an optimum reflow recipe. Such software can be used to identify a common recipe that can process groups of different PCBs. One method is to simply attach a large and a small PCB to the same profiler. The software will search for a recipe that can process both PCBs within specification, so that PCBs between the two limits will be in specification. Depending upon the reflow oven, a single recipe may suit all PCBs. Software can also help, designing a handful of recipes to serve small, medium, and large PCBs. The use of TCs can verify the effectiveness of these reflow oven recipes.
Automatic and continuous profiling that measures the profile in real-time for each and every PCB being produced is becoming popular among many manufacturers
Prediction software for reflow ovens can identify a new oven recipe by only changing the conveyor speed, and keeping the zone temperatures the same. Changing reflow oven recipes via conveyer speed is near instantaneous. But if the temperatures of a reflow oven recipe change, an oven may require 30 to 45 minutes to stabilize to new temperatures, especially when changing from hotter to cooler temperatures. Reflow ovens will heat up much faster than they cool down, and some production managers may schedule production runs during a day by starting with the coolest reflow oven recipe and ending with the hottest. This approach requires more pre-production planning across the organization, but enables a higher mix environment to become higher volume by improving efficiency, noted del Valle.
High Reliability Electronics
Many electronics manufacturers, especially those involved in high-reliability (hi-rel) and automotive electronics, require the use of best manufacturing practices. The reflow oven must be set up correctly and verification profiles run to ensure proper operation. For such products, clients and manufacturers are constantly in discussion regarding the required frequency of profiling. Opinions can vary greatly on how much profiling is enough, since some feel modern reflow ovens are stable and require less profiling while some feel that there can never be enough accurate knowledge about the characteristics of a reflow oven and a solder reflow process.
Common variations like changes in the exhaust system, thermal loading, pressure changes, and human errors may affect the thermal process significantly. Automatic and continuous profiling that measures the profile in real-time for each and every PCB being produced is becoming popular among many manufacturers because the technology provides traceability, eliminates the problem of running blind in between spot checks, and eliminates the production downtime associated with periodic manual profiling. The systems will further reduce downtime by monitoring the oven during the changeover phase and alert the technician the moment that the oven is ready for production.
Another source of downtime can be the time required to troubleshoot a production run that has stopped due to a yield problem. Although typically less than 5 to 10 percent of defects are attributed to the reflow process, many factories will run a profile to troubleshoot a yield problem. This is due to the lack of process information about the reflow oven. Automatic profiling will inform the engineer immediately whether the problem is in the reflow process or not, reducing troubleshooting time. If the problem is in the thermal process, the system provides information to identify the problems in the oven.
Automatic Profiling
It is not uncommon for SMT production lines to spend more time on setup and line changeover than actually running production. Additionally, following best manufacturing practices for the reflow process may not be possible or acceptable at these factories. Today, process engineers and managers are doing their best with what they have. They can use techniques like scheduling the production batches to run cooler oven recipes in the morning and progress towards hotter recipes at the end of the day. To significantly improve profitability, however, high-mix/low-volume manufacturers are taking advantage of new technologies such as prediction software.
Oven setup and process optimization software may improve both productivity and quality by allowing for a more scientific and accurate approach to reflowing the assemblies under such difficult circumstances.
NuVal EMS achieves high yields and low costs by ensuring that every product it manufactures is well designed, properly laid out, and in concert with the client. Every product is thoroughly tested before going into production.
Because customer service and personalized solutions are important, NuVal EMS focuses on providing customers with the highest possible levels of quality and productivity.
As del Valle admits: “There are hundreds of contract manufacturers to choose from, but there are very few that do what we do: focus exclusively on the customer.” He added: “At NuVal EMS, we don’t want to just churn out products for clients we may never see again. Our focus is on developing long-standing relationships with clients where we become an extension of the client’s engineering and operations team. We want to take on customers’ most difficult challenges and turn their product sketches into trailblazing innovations.”
To find out more about NuVal EMS, contact Jose del Valle, COO/ Vice President, at
14831 Myford Rd., Tustin, CA 92780; 714-544-0105, E-mail: [email protected], Web: www.nuval-ems.com.
Contact: KIC, 16120 W. Bernardo
Drive, San Diego, CA 92127
858-673-6050
E-mail: [email protected]
Web: www.kicthermal.com
Reprinted with permission from the August 2015 issue of US Tech