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Using the Internet with Real-Time Reflow Data Collection for Process Monitoring and Quality Control

Mike McMonagle, K*Tec Electronics
Sugar Land, Texas

Abstract

In today's competitive SMT manufacturing environment, the ability to monitor the reflow process in real-time has become critical. Failure to obtain accurate process data can have a disastrous effect on yields -- an unmonitored, out of control reflow process can cause defects on every board that passes through the oven. These failures may manifest themselves immediately, requiring expensive rework; or may only reveal themselves in the field, causing end-user and customer dissatisfaction. At our contract SMT assembly facility, we have virtually eliminated these risks, as well as rework due to reflow defects, by implementing a program which captures data from the reflow ovens of our production lines via a real-time thermal management system. As a further productivity improvement, the data gathered by this system is available for remote monitoring of the process via our company intranet.

Profile Criteria

As a contract manufacturing facility, we produce a wide variety of assemblies and must therefore address a wide range of factors when creating an optimum profile. A tightly controlled ramp rate is one of the most critical profile characteristics. A relatively small variation in degrees per second of product temperature ramp rate can undermine the overall integrity of the components and cause defects. In the soak zones, variation in product sizes requires a broader range of profile settings, as smaller or larger products take different times to reach their pre-reflow temperature equillibrium. In the peak zone, a tight window is more critical than in the soak zone but not as critical as the ramp rate. Maximum temperature and time above liquidus is also important. Our processes require a nominal peak temperature of 215°C and a maximum of 225°C. Minimum time above liquidus must be at least 30 seconds and must never exceed 90 seconds.

These statistics are based on input from paste and component manufacturers, as well as our own in-house process experience with various component lead frame finishes such as palladium, palladium/silver and Alloy 42; and alternative surface finishes including OSP, immersion gold, and immersion tin. Though some of these do not require a tighter process window, they can require more robust profiles, especially in terms of peak temperature. Proper profile development will only increase in importance as we move forward into the use of other solder alloys being brought about by the current lead-free legislation and development in the European communities.

The Real-Time Thermal Manager

The installation of a real-time thermal management system on all of our reflow ovens made it possible for us to get real-time live data output from processes that have traditionally been difficult to monitor. The real-time thermal manager continually monitors process temperature in the reflow oven. Thirty thermocouples embedded in two slim stainless steel probes are permanently mounted just above the conveyor. They are mounted close enough to the board to provide representative temperatures, but far enough from the oven rails so as not to be influenced by the thermal mass of the rails themselves. A representative example of an oven installation is shown in Figure 1.

Figure 1 - Thermocouple Rods Mounted To Oven Rails

Though the system does not actually measure the board temperatures, it provides a detailed measurement of what the process temperatures are at the level of the board as it passes through the reflow oven. This is a much more accurate representation of the actual reflow process than the data provided at the thermocouples within the blower plenums, as illustrated in Figure 2 on the following page. The creation of a baseline of these thirty data points during initial profiling can be used to monitor and accurately determine if the qualities of the original profile are being adhered to during actual production runs.

Figure 2 - Comparison of Oven TCs vs. System TCs

The probe thermocouples continuously monitor the process temperatures, taking readings as frequently as every five seconds. These temperatures are then processed by a dedicated controller and displayed as "Process Profiles" on the oven's PC screen. All data from each reading is recorded permanently to the hard drive, giving users the ability to review process data from any previous production date and time. This capability to 'turn back the clock' and view data from previous production runs is just one benefit of the system. Using this feature in conjunction with our barcode-based, real-time quality control data collection system allows us to document the thermal profile of any given board in a production run. This also provides assurance that the assembly or components were within their specified parameters when addressing component related assembly failures with component manufacturers.

The real-time thermal manager provides a product profile for every board processed by creating a mathematical correlation between product profile, as measured by a pass-through profiler, and process temperature, as measured by the real-time thermal manager thermocouple probes. This 'Virtual' product profile is calculated every 30 seconds, and Virtual Profile statistics, such as peak temperature, are also calculated and continuously updated. The real-time thermal manager can detect critical temperature variations that oven control thermocouples cannot, and immediately reveal these temperature drifts and their location. The instant the Virtual Profile falls outside of the pre-defined process window, an alarm will sound. This feature can also turn on an alarm light, or even shut down the oven input feed conveyor to prevent any further assemblies from being processed. Because it eliminates the potential for product defects due to thermal variation, the real-time thermal manager is an excellent tool for ongoing monitoring of the reflow process.

Other Benefits Of Thermal Management

The systems utilized on the reflow ovens are real-time in all aspects, during both initial profiling and production process monitoring. This adds additional value and ease of use during initial profiling and adjustment. The pass-through profiling unit provides nine separate thermocouple inputs to allow monitoring an adequate amount of components and a variety of assembly locations on even large, complex assemblies. It is a wireless system that provides live output to the profiling software throughout the profiling operation. The components consist of: 1) the profiler, containing the transmitter and a high-temperature trailing antenna lead; and 2) the receiver, which interfaces to the dedicated controller. The wireless profiler and receiver are shown in Figures 3 and 4 respectively.

Figure 3 - Wireless Profiler Attached To Profile PCBA

Figure 4 - Receiver Unit Mounted at Reflow Oven Entrance

This wireless system speeds profile development by eliminating the need to capture profile data in the profiler's memory and then return to a desktop computer to dump the data and review the results. If a problem is found during the profiling due to bad thermocouple attachment or gross temperature errors in the profile, the profiling can be aborted; thereby saving time and preventing possible damage to the assembly or profiling unit itself. The real-time profiling data output screen is shown in Figure 5 on the following page.

Figure 5 - Real-Time Profiler Data Output Screen

During initial profile creation for a given assembly, much time can be consumed in multiple passes of the assembly through the oven to fine-tune and optimize the profile. The real-time thermal manager provides a software option to reduce this time without decreasing the accuracy of the profile. The software option is a predictive model that allows an initial profile using 'generic' oven zone and belt speed settings. We can then review the profile plot and data, and instruct the software in any changes we may want to see in reduced ramp rate, changes in soak time or temperature, peak temperatures or other thermal parameters. Based on these changes, the predictive model reviews hundreds to thousands of possible scenarios to achieve the desired results. It will then present them, ranked to provide the best results and widest process window. The ranking algorithm, known as the Schultz Index, assures that the profile changes selected fall well within the center of a bell curve ranking. An illustration of the bell curve model is shown in Figure 6 below.

Figure 6 - Schultz Index Bell Curve Ranking Model

Upon reviewing the suggested changes, you can then choose the most appropriate profile based on ease of profile changeover from one profile to another, faster throughput based on belt speed or other possible needs specific to your process situation. The screen display of the Auto-Predict Index Ranking function is shown in Figure 7.

Figure 7 - Auto-Predict Ranking Selection Screen

After inputting the changes to the oven, a second pass of the board is performed to validate that the changes have resulted in the desired profile. This allows an optimum profile to be created rapidly, without tying up valuable equipment and engineering resources. In the majority of cases, two passes are all that is required to achieve an optimum profile.

The system software is a key component of any thermal management system. Our current system allows the setting of different access levels, each custom tailored to the features needed by the individual using the system. This provides rapid learning and use of the system by entry level process technicians who would otherwise be challenged by the variety of options available within the software. This also eliminates the possibility of setting their setting profile parameters to yield what they might consider an acceptable profile rather than following established department process guidelines. More experienced engineering personnel have access levels established allowing unrestricted changes to any and all profiling criteria. With this, profile parameters can be set to allow evaluation of new materials and processes and conduct other design of experiment exercises.

We have developed Virtual Profiles for each of our products. The Virtual Profile gives us real-time monitoring well above and beyond the simple heater core thermocouple data provided by the oven controller. The real-time thermal manager provides us with twice the number of thermocouples found in a typical oven, and these thermocouples give us data on what is happening to the process at the belt rather than at the heater element. This tool yields much more finite measurements than the oven controller thermocouples and allows us to fine tune the process to each specific assembly. The real-time thermal manager can rapidly provide feedback based on the setup of the oven, verifying that our process has been properly set up to fall within the parameters we've established for our reflow profile for a given assembly. The Virtual Profile data screen also provides a small colored 'crystal ball' indicator icon at the top of the display screen to give a quick indication of the process quality. If it is running within the predetermined setpoints for the profile, it will display a green icon. If it is outside of the optimum, but still within the acceptable temperature range established, it will indicate a yellow icon. If outside of the acceptable range, it will indicate red and activate any additional alarm functions programmed.

Network & Intranet Overview

Our corporate network and intranet provides a variety of data processing and distribution services. Standard mainframe applications such as production planning, procurement and accounting are deployed and utilized throughout the corporation. In addition, specialized intranets are established for individual corporate divisions. The intranets support a broad range of data and documentation services, including human resources, manufacturing, engineering and quoting. Our divisional intranet is also used for our real-time quality reporting and in-circuit test reporting systems, which are available throughout the company to anyone with intranet access and accessible by outside customers via their own websites on our server.

The corporate network data center resides within our facility in Sugar Land, Texas. It currently supports over twenty-one hundred users, with approximately nine hundred of them residing within our contract assembly division in Sugar Land. The corporate network data center provides data and networking services to four distinct corporate divisions with a total of sixty-one physical locations throughout the country. Hardware redundancy and automated off-site data backups provide a high level of data integrity and an overall network uptime rating of 99.9%. Disaster recovery capability includes fiber optic and standard copper duplicity, along with satellite data transmission. Finally, the entire facility is constructed to withstand a Force 4 hurricane, with on site power generating capability to run all operations (including full manufacturing) for up to seven days.

The backbone of the system is handled by 5 frame relay connections provided through 2 national carriers, along with one point-to point T1 connection. Frame relay networking provides distinct advantages over dedicated high-speed lines in that a variety of network topologies can be easily created via software. It also provides bandwidth on demand to allow constant load balancing throughout the entire network, based on current requirements at any given time. Frame relay provides additional efficiencies by executing error checking routines at only the sending and receiving end, as opposed to performing them at each switching node along the transmission. This translates to higher transmission speeds within the network, with reduced network response times improving the response times of applications such as real-time process monitoring that traverse the wide area network.

The frame relay connections are dispersed via a 'cloud' network topology to the various divisional site locations, and are connected to at between 56k and T1 speeds. Total network speeds vary between 1.024 to 1.536 Mb/second, and are fractioned and continually adjusted by load balancing.

Real-Time Thermal Data Distribution

The data from the real-time thermal manager is distributed to remote locations via the division intranet. The thermal manager's real-time output is captured at 15 second intervals by a third-party screen capture program at the thermal manager PC, and all of the thermal manager PC's are tied into the internal network. At a pre-determined interval, the screen capture program exports the Virtual Profile image to a folder on one of our servers. The process data from this folder is retrieved and displayed by a web page on our corporate front end. The page is setup using a meta-command statement which allows the web page to go out to the folder, grab that image at an interval that matches the snapshots, and then automatically integrate it into the web page. As the data is captured by the screen capture program and placed in the folder, it automatically overwrites the previous image to avoid eventually filling up the server hard drive.

This system allows viewing data from either an individual oven or multiple ovens at any given time and will have the capability to display multiple ovens on the production floor in a single snapshot window. The ability to access any or all ovens on the production floor greatly reduces the time engineers need to be away from their desks. The information from the output folder on our corporate intranet can also be accessed at the same interval by an internet website we've established for this specific purpose, which is shown in Figure 8.

The website screen includes a header designating the line and oven type, as well as a snapshot of the live data output from the real-time thermal manager. The live data output captured is structured to display the oven footprint with zone temperatures and belt speed, along with average and maximum deviations. A statistics block showing ramp rates, soak time/temperature, time above liquidus and peak temperature can also be included. Also seen is the Virtual Profile 'crystal ball' icon, indicating green, yellow or red for whether or not the process is running in control.

Figure 8 - Intranet Output of Real-Time Data via website

The main concern of output of the data via the internet versus our corporate intranet was the refresh response time increases due to the transmission routing delays experienced going through multiple internet servers. An experiment was developed and conducted in conjunction with the manufacturer of our thermal management systems to validate the feasibility of ongoing internet transmissions, with successful results. Using a 128k ISDN connection, we were able to transmit real-time thermal data from our facility in Sugar Land, Texas to a major industry trade show in Southern California earlier this year. The data was viewable in real-time the entire length of the show, with the image quality and refresh response rates considered more than acceptable for our purposes. Based on this, the decision was made to begin full implementation on a target assembly line for further test purposes. We are now in the process on extending this system throughout the remainder of our reflow systems. We are also now achieving acceptable external data refresh rates utilizing a standard 56k dial-up connection. The representative internal and external data flow routing is detailed in Figure 9.

Figure 9 - Diagram of Real-Time Monitoring via Intranet Network

Effects Of Distribution

The effect this internal distribution has on production costs and quality control cannot be overstated. With qualified engineering staff difficult to come by, many companies are running lean on engineering and support staff. Being able to access data remotely eliminates the engineer's need to constantly jump up and go out to the shop floor to view each process. The internal distribution of data from the thermal manager via the intranet has allowed us to maximize the value of scarce engineering resources. Time spent efficiently by engineers means a better bottom-line for any facility. The key to our success lies in the real-time thermal manager's ability to provide quick and easy access to the process status, which makes it simple and convenient to determine whether our process is in or out of control at any time.

The external distribution of the thermal process data to our customers via the internet will allow us to further increase customer satisfaction, and continue to demonstrate cutting-edge technology. All customers currently have their own individual web sites on our corporate intranet, which they can access through our firewall with a personal password. Within their respective sites, they are able to view work in process and real-time quality data collection showing both in-process inspection and test yields. Their ability to dial in and check out their particular product at any time, from anywhere, assures them that they are getting a quality product that is being built to their specifications.

The steps we have taken to monitor our reflow process and make the data available over the web are significant steps towards monitoring our entire process remotely and in real-time. We will continue to move forward in implementing such remote monitoring capability in other assembly processes, including wave solder, screen printing and pick and place equipment. The value of this advancement should be readily apparent to engineers and technicians involved in the electronics assembly industry. Installation of the real-time thermal manager on our reflow ovens and wave solder machines has greatly improved our process by reducing defects and increasing yield. The further benefits of distributing real-time thermal process data over our intranet and the internet has increased the efficiency of our in-house operations and gives our customers peace of mind regarding this critical portion of the assembly process.

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