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July 26, 2010

SPC for Reflow Ovens

What is the best way of running SPC on a reflow oven?

K. F.

Experts Comments

What most people mean when they ask, "what is the best way of running SPC" on most any process is, "what is the most practical way?" This is because SPC math demands a lot from a process.

In fact many SPC tools, like GR&R, cannot be done for a reflow solder thermal process. This is because the GR&R study demands you measure the same "part" more than once.

People assume this means you run the reflow solder profile a second time, which does NOT measure the same part a second time, it produces a second and completely new part, which is not the same as measuring the same part two times. See more discussion on this in my blog.

SPC also demands a lot of data, and even the simplest profiling tools take a little time and discipline to perform. So there is often too little data to allow SPC to really work correctly.

However, corporations willing to take the time to get the needed data from their reflow process will gain much in improved process control, proof that your oven is in control, increased yields, reduced scrap, as well as a marketable differentiator over your competition, in that it shows that you are concerned about your thermal processes.

The most practical way to collect data about your reflow oven, and thus begin to produce meaningful process SPC, is via tools which are designed to measure your oven's capability at a given recipe (oven settings).

These tools have the advantage of remaining stable over many 1000s of uses and thus allow you to see how your oven is performing over time. Unlike the original circuit board, which was used to develop the oven recipe, these tools can confirm that the oven is repeating the same thermal environment long into the future.

They can be use to check if your oven is ready to run at a production recipe at each shift change, every day, after each recipe change, etc, and at the same time capture valuable data which can be used to produce the SPC control and capability charts for the reflow oven.

There are several of these tools to choose from, like the OvenRIDER by ECD. Much more information than can be included here can be found in my blog on this very question of SPC for reflow ovens (Oven Verification) using an OvenRIDER. Please feel free to take a look.
Paul Austen
Senior Project Engineer
Electronic Controls Design Inc
Paul been with Electronic Controls Design Inc. (ECD) in Milwaukie, Oregon for over 34 years as a Senior Project Engineer. He has seen and worked with the electronic manufacturing industry from many points of view, including: technician, designer, manufacture, and customer. His focus has been the design and application of thermal process measurement tools used to improve manufacturing processes like: mass reflow and wave soldering, bread baking, paint and powder curing, metal heat treatment and more.

The first question to decide upon is whether you want to run SPC on the machine itself (reflow oven) or on the process. The reflow oven will constantly change in order to maintain a stable process.

The oven is trying to compensate for variances in load, ambient temperature, preventive maintenance, wear and tear, flux build up, and much more. It therefore makes more sense to run SPC on the process itself (profile), which is really what determines your production quality.

There are a few accepted methods to run SPC on the process.

  • Run a profile on a PCB at set intervals such as every shift, every week or other. The profile data may be sent to an Excel spreadsheet or SPC program for charting. Most thermal profilers will have optional SPC programs that will do this for you.
    The problem with this approach is that a PCB will deteriorate already after a dozen runs. Changing to a new profile board will introduce unwanted changes to the measurements.
  • Use a fixture that can last hundreds or thousands of runs. This is usually a good and inexpensive method. Make sure that you use fixtures that accurately represent the profile on the PCB.
    The drawback to this method is that running profiles with fixtures is a manual method that limits the number of measurements and prevents real time feedback. It can be labor intensive and interruptive to the production.
  • There are a few automatic profiling systems on the market that provide automatic SPC charting without any human intervention after the initial programming.
    There is an up front investment in these systems of a few thousand US$, but they have a number of benefits:
  1. Real time Cpk data will typically warn instantly of an out of control process while the oven is still producing in-spec products, hence allowing the technician time to adjust the process before defects are produced.
  2. No interruption to the production
  3. Virtually no labor cost associated with collecting and displaying the large quantity of SPC data.
Bjorn Dahle
Bjorn Dahle is the President of KIC. He has 20 years experience in the electronic manufacturing industry with various manufacturing equipment companies covering pick & place, screen printers and thermal process management.

One of the easiest ways of running SPC on a reflow oven is to use our ECD's OvenRIDER, whereby the product software automatically capture the necessary data and plot the SPC chart.

You just need to run the OvenRIDER through the reflow oven at a same recipe over a desired number of cycles.

EH Lim
Managing Director, Asia Pacific
EH Lim has been in the PCB Assy industry since 1985, starting at Thomson/Singapore for 5 years before moving to Electrovert Asia Pacifc. Lim was Sales Director for Vitronics Soltec prior to joining ECD in 2007 as Managing Director for Asia Pacific.
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