The Importance of User Objectivity in Process Control: Article

Charlie Fujikawa
Senior Sales Engineer

Over the past 10 years, the SMT industry has made many technological breakthroughs and improvements to the production process. Chips, packages, and BGAs continue to get smaller, with finer pitches. The equipment continues to get faster with increased precision. Now, more than ever, there is an increased need for manufacturing sites to increase their productivity and efficiency. One integral step in increasing productivity, is to decrease losses due to errors. This may seem like a no-brainer, but it is often easier said than done. Let’s look at how process control can help, and often solve our quest to limit production errors.

Malcom PCU-285

All manufacturing sites have some level of process control. This is the process in which we can ensure that the components of the production are working optimally. When we find errors in the production, we can trace the source using process control to eliminate different factors that can contribute to the error. Eliminating trouble areas in these key points in the production allows for increased efficiency in troubleshooting. This is were we come across a very common problem with a lack of process control. Many manufacturing sites depend on operator subjectivity to determine whether a process is acceptable or not. As the operator changes from site to site, or even shift to shift, there is no standard to judge the process objectively. Let’s take a look at one of the major components of the manufacturing process, and the effects of proper process control over it.

Solder paste is one very important component of the SMT process, and it can be a key factor in production efficiency. One major aspect of solder paste that is often overlooked is the paste viscosity. There are a number of different manufacturers producing solder paste for SMT production. All the paste manufacturers use a standardized testing method (most commonly IPC or JIS testing) to assure their paste meets an industry standard for production. Each paste is created and verified to be used within a specific viscosity range for optimal production conditions. Once the paste is shipped and refrigerated, the viscosity and condition of the paste changes. Viscosity is directly related to the temperature of the paste. In many cases, when the paste is ready to be used in production, it is taken out of refrigeration and needs to be conditioned for use. In an uncontrolled process, an operator with take the paste out of the refrigerator, and leave it out to thaw for at least a few hours, waiting for the temperature to reach 25˚C (standard production temperature). The time is dependent on the temperature and conditions of the manufacturing location. Next the operator will use a spatula to manually mix the paste till it “looks” or “feels” mixed and ready to use. This process is dependent on the operator’s judgement of the paste’s condition. How do we know the paste is at the optimal temperature, viscosity, mixture, and condition for production? If the paste is too viscous, the material can stick to the stencil and leave divots, canyons, and/or peaks in the solder pads. If the paste is not viscous enough, the pads may not hold their shape and slump, leading to solder bridging or low wetting tension/force. Production runs could yield different results depending on who is making the judgement calls on the solder paste conditions. To help battle this, as well as troubleshoot issues, many manufacturers are requesting traceability data be recorded for all stages & processes in the production.

Malcom SPS-2000 Solder Paste Mixer

If we introduce a level of automation and testing to this process, we can be certain of the solder paste’s readiness for production. An automated paste mixer, such as the Malcom SPS line of paste mixers allows the operator to take the paste out of refrigeration and prepare it for production in a fraction of the time. The paste mixer will not only fully mix the paste, but it will also bring the paste up to room temperature. The user will not have to worry about air bubbles, flux separation, or inconsistencies in viscosity or temperature. Once the process has been set, it can be reproduced with the same results every time, regardless of operator skill. Solder paste can be ready for production from refrigerator to line in about 10min. In order for us to verify that the paste is at optimal viscosity and temperature, a Malcom viscometer can be used to verify the paste. The Malcom PC-11 and PCU-200 series viscometers are the same ones used by the paste manufacturers to measure paste viscosity. Measuring these readings before production allows for an actual objective reading to be recorded as proof, and to provide traceability. Spot checking the solder paste from time to time during production also adds an extra degree of verification to the process. This assures that the paste is still within the recommended viscosity range as the production cycle progresses and also calls for more paste. The longer solder paste sits on the stencil, the more the properties of the paste change. This process is a great way of confirming that the solder viscosity is not the problem. Moreover, we are eliminating inconsistencies that arise due to operator variability. We can record these readings so we can have objective data to trace the paste conditions at any given point in the production cycle.

Of course, solder paste viscosity is only one factor that can contribute to errors within the manufacturing process. As you can see, reducing operator variables by automating processes; as well as verifying a process by measuring and collecting data goes a long way in improving your process control. If errors are found at a higher rate during a specific production run, those boards can be cross referenced to data to verify the specific process was within spec or not. We can set-up the same sort of process to reduce errors caused by other factors, such as bare PCB cleaning, PCB/part/reel humidity, reflow oven conditions, and stencil cleaning. Seika Machinery, Inc. offers many different tools and equipment to help with process control. Examples of some of these tools are: Malcom viscometers, paste mixers, wetting testers, reflow oven profilers, McDry low humidity storage, Unitech PCB cleaners, and Sawa ultrasonic stencil cleaners. These are just some of the equipment we offer to improve your complete production and testing processes.