Calibration of electronic measurement instruments is a necessary process, even though most electronic equipment is very stable and somewhat “resistant” to the effects of environment and changes due to aging.

Q: So why calibrate if my MOLE is “in spec” every time I send it in for calibration?

Because calibration is not so much an adjustment process but rather a proofing process that shows, over time, that your MOLE has been in calibration and thus should remain in calibration, because you have a track record to prove it. Documented history of a MOLE’s performance is the only way to claim your MOLE is in calibration at any given instant.

Most good labs will tell you that when your MOLE is calibrated, it is compared to standards , typically standards that have traceability to NIST, and if it is shown to be measuring within its specified accuracy they will not make any attempt to “adjust” it. Only if it is “on the edge,” which usually means it is getting to the last 10% to 20% of the specified accuracy limit, will they make any adjustments. Your MOLE may still be “in spec” and thus “in calibration,” when the lab received it, but getting close, so they will adjust it closer to the middle of the spec. range.

If it is out-of-spec when received by the lab, then a red flag goes up and calls into question every measurement made since the last calibration! The lab will tell you how far out of spec it is, and you can decide if its measurements during that time affect the quality of the measurements made more than can be tolerated, or if they are “close enough” to still be acceptable.

Q: So, when should the MOLE be calibrated?

The number one best time to calibrate the MOLE is on a regular time-based interval, which is recommended once a year. However, there are other events which may cause you to want to seek calibration at other times of the year, such as:

1. When the MOLE is subjected to rough treatment like a fall to the floor,
2. When your MOLE is accidently “over heated” ,
3. When you are starting a new product introduction and you are characterizing an oven and new assembly to find the right recipe,
4. When a new customer’s contract stipulates you use equipment that has been recently calibrated,
5. When your in-house quality program requires a calibration interval.

Getting your MOLE calibrated is easy and we want to make sure you are always making the highest quality temperature measurements.

For example, component manufacturers would have you avoid certain limits in temperature or temperature change rate (slope) to avoid damaging their parts. And that’s ok, but that only tells you what to avoid, temperature wise. Most solder paste manufacturers would have you believe that their paste can take most any reflow thermal process so as not to be excluded from purchase. This too is understandable and in reality, most solder pastes are good and will solder your components to your circuit board. Many standards (like IPC standards) on the subject suggest what your product MUST withstand to be considered robust and not necessarily an ideal reflow process thermal profile. This makes sense, because there is no one reflow thermal profile that will solder every possible circuit board assembly, and standards must be general in their application. Then there are the public websites that are often peppered with bias toward a specific brand of profiler in their description of what’s important or how to view it.

Each year new talent enters the work force and training in the art of reflow soldering is limited or costly. Worse yet, some learning about the reflow process only occurs from failures caused by incorrect reflow process settings. And perhaps worst of all, many reflow solder machine are still running the same setting set generations ago because no one currently available has the skill to make them better. Just because many industry veterans understand the issues around reflow soldering and thermal profiling does not mean the new talent can hit the road running. And, since most every electronic assembly will pass through either reflow soldering, wave soldering, or sometimes both, I thought it important to take a look at the reflow solder process, dissect it and consider what’s important to measure and control. Click here for the more in-depth look.

1. “I’m from the Metrology lab and it’s time for the annual calibration of your reflow oven.” We’ve got an app for that.  After you’ve finished with the oven’s calibration procedure, you can run the MegaRIDER-20 with a Process Test Pallet to see if the machine is uniform across the conveyor width and has the same heating capacity as it did the last calibration or maintenance.

2. “I’m the Manufacturing Engineer and our QC Department wants me to show that this oven is in control.” We’ve got an app for that. You probably need more information than the once a year Metrology profile can provide. So weekly you can an OvenRIDER and see that every zone in the oven is performing the same using X-Bar R charts to prove it.

3. “I’m on the New Product introduction team and I need a good recipe to solder a new board without killing the parts.” We’ve got an app for that. The Super M.O.L.E.® Gold thermal profiler will let you connect T/Cs to the board to see exactly what’s going on, thermally, on the areas where you and the designer have the most concern. Use the Prediction tools in the new MAP software to lock in the perfect recipe.

4. “I’m a Line Technician and I have to know my reflow oven is ready to run product without all the wires and circuit board stuff.” We’ve got an app for that. OvenCHECKER is one pallet loaded with the most powerful profiler on the market today. It takes no more time to run than the first production board and it lets you know if the reflow oven is ready or not. No downloading, no comparing numbers on a chart, just Go, or No-Go.

The ECD V-MOLE with patented one button “OK” profile evaluation

(The OK Button is also available on the 20-channel MEGAM.O.L.E.™ thermal profiler, and OvenCHECKER™ )

Taking only seconds, the MOLE can tell you and your oven operators if the profile just measured is in or out of specification with the universally understood Green for good (GO!) or Red for bad (STOP!).

You get to set the specification limits for any or all of the four most popular profile parameters:

• Ramp Slope
  
• Time Between temperatures
  
• Time above Liquidous
  
• Peak Temperature
  

…and you can choose which of the MOLE’s input channels to include, up to 20 channels on the MEGAM.O.L.E.™, and three on the V-M.O.L.E.™ thermal profilers.

The Specification Table in MAP Software

Using the MAP™ Profiling software, enter your specification limits for the four profile parameters in the Upper and Lower Limits table in the “Target-10 OK” tab. These values will automatically be sent to the MOLE profiler when you use the Verify Process Wizard to confirm that a previously characterized oven recipe is still performing within specification.

MAP™ Profiling Software Target-10 OK Profile tab

Once programmed, the MOLE profiler can be used many times (up to 96 times) to Verify your oven is producing the same profile, without reconnecting to your computer. Simply run the profile and press the “OK Button” on the MOLE. No more running back to the PC software to download to see the results. One push of the OK button, and you get your answer…Go, or No-Go. It’s that simple!

*U.S. Patent Number 7653502.

You might ask, why should I perform an Oven HealthCHECK? In other words, why should I run a rather sophisticated measurement system through my oven to produce a rather nice looking 3-D plot of the cross belt temperature uniformity?

Figure 1: An example 3-D plot of the oven with very good cross belt uniformity, < 3ºC

I may not like the answer because I might find out my oven has a problem that I cannot fix? Sounds like a “head in the sand” sort of excuse to me.

I would ask a different question: “Why do you calibrate your bench test instruments?” Is it to find out that the instrument is out of spec? Heavens no!! That would be a nightmare because it would call into question everything that instrument was used to test since the last time it was calibrated. So why do you calibrate if the results could be so disastrous? Simple, it allows you to show and document that the instrument is and always has been “within specification.” So when the auditor asks, how do know your instruments are in calibration, you pull out the Certificates of Calibration.

Figure: What’s the first thing you check after you receive your Certificates of Calibration? The “As Received: Within Tolerance”

ECD’s Oven HealthCHECK is designed to certify and document your oven’s performance or health. It is a “calibration” done on a regular interval, say once a year, to show that the oven performance is within specification and not changing over time. It can also provide a baseline level of performance around which you can compare into the future. Further, if you have several ovens, the HealthCHECK can show you which ovens are best for applications where oven uniformity is critical.

Back this up with much simpler and more frequent verification profiles of your oven using OvenRIDER or OvenCHECKER, where simple software-generated Xbar-R control charts show daily indications of a thermal process that is “in control,” and you will no longer have to steer the auditor around your reflow oven. You can proudly show that you know your oven’s performance level and that it is consistent because you have taken the steps to measure your oven’s health as part of your Thermal Quality Management program. Such a program should be marketed, since it shows you commitment to understanding your oven’s thermal nature and you have the data to show it. This sure beats the “head in the sand” quality program which may characterize your competition.

So let’s take a look at the values produced by a typical G R&R study and then see how they could fit with the data produced by the OvenRIDER® or OvenCHECKER™ software:

Appraiser Variation (AV%) – This is the percent of the total variation which we humans introduce because the measurement method may vary due to physical or subjective interpretation issues. Things like value interpolation between two tick marks on a ruler or how hard they squeeze a caliper around a part. This is the “Reproducibility” part of a measurement. The lower the measurement variation caused by the appraiser, the better its reproducibility and so almost anyone can do it and get the same or very close answer. If it is more than 30%, then there is too much variation from one person’s measurements to another’s. You will want to make sure they are following the procedure and not taking some shortcut.

Equipment Variation (EV%) – This is the percent of the total variation which is caused by the measurement equipment or tools used to make the measurement, like a ruler for length or thermometer for temperature. This is the “Repeatability” part of a measurement. The lower the variation caused by the equipment, the better its repeatability. Again, more than 30% means a good portion of the variation lies with the measurement tools.

Reproducibility and Repeatability (R&R%) – This is the combination of the AV and EV and is the percent of the total variation due to the tools and the people used to measure the parts. The measurement system, people and tools should produce a percent of less the 10% to know you have a good measurement system. Between 10% and 30% and you should work to improve the component(s) contributing the majority of the variation.

Part Variation (PV%) – This is the percent of the total variation which is caused by the parts being measured. This is where most of the variation should be found no matter how small the variation. After all, the part variation is what you are trying to measure, no matter how small. So expect to see this number in the 90% range, which means your parts are what is varying.  

Total Variation (TV) – The TV is typically not given in percent and is a “standard deviation” like number where one can expect 99.73% of all the measurements to fall between +/-3 times the TV.

The form shows a typical example of a GR&R with sample data one way the values can calculated. How the numbers are calculated is not too important at this point. What they mean and how one can apply them to the data captured from an OvenRIDER® or OvenCHECKER™ is the real question.

So what does it take to do a GR&R?

1. Two or more Appraisers – These are the people who are going to use the measuring tools to measure the parts.
2. At least 5 sample parts – These are the things the manufacturing process is designed to produce.
3. At least two trials measurements – This means measuring the same value from the same part at least two times.

How do these three things map into to the data produced by the OvenRIDER® or OvenCHECKER™:

1. We have two or more appraisers, no problem!
2. How about at least 5 sample parts? Well, what is the “part” we are making? We have an oven, and the oven has a recipe which was characterized to melt solder without killing the parts on your boards. So, the oven is set up to make … thermal profiles, or time vs. temperature graphs of a specific shape. We assess the shape in many ways, just like we could assess a bolt made by a screw machine. On the bolt we could measure its length, diameter, thread pitch, hardness, etc. On a profile, we measure the initial ramp slope, soak time, time above liquidous, peak temperature, etc. So the profile is the sample part, and one of the profile measures is the value we want to study. Let’s say, peak temperature. Then you run the RIDER through the oven at least five times to make at least 5 sample parts (profiles). So far so good.
3. Now each appraiser is to take at least two trial measurements of the same value off the sample parts. So for a given profile, instruct each appraiser to measure the same value off the part (profile) at least two times. How? The software, which takes the data out of the profiler which was recorded from the RIDER, extracts that data from the profiles automatically. No matter how many times or who you instruct to ask the software to give you the peak temperature for a given profile, it will produce the exact same value. If you wanted to do it the hard way, and have each appraiser look at the profile graph using a straightedge and pen, and interpolate the peak value from the graph, you would likely get some appraiser caused variation. But no one in their right mind would do this.

What does this mean? It means that no matter what version of GR&R you use to run the math, the AV and EV will be 0%, leaving all the variation in the parts, which does not make for a very practical GR&R study.

But wait, could you just run the RIDER through the oven again to take a second trial measurement of the SAME profile (or part)? That is NOT a second measure of the same part. That would be producing another new sample part, not a second trial measure of the same part.

This is a typical problem with over half of the measurements we are asked to take in industry these days. It can only be measured once, and then it’s gone. If you want to pull to failure test a bolt as the GR&R studied value, you can only do it once, and then the bolt is broken. You can’t put it back together to make a second or third trial measure. It is the same with a profile. It happens in time and once the profile is produced, that specific profile at that instant in time cannot be measured again. It’s gone! You can only produce another profile (or part), which is not the same as measuring the same profile value twice.

So GR&R is not really the best method of assessing a RIDER product’s variation. The real question is, what is the accuracy of the instrument (the MOLE® thermal profiler), which is stated in the specification, and the variation in the RIDER sensors?

Great care is taken to manufacture sensors to a tight tolerance and tested to show that they have consistent response time to temperature changes. That, coupled with the fact that the materials do not break down or alter properties with use, makes it reasonable to accept any variation measured by the RIDER products lies most completely with the machine producing the profiles.

References and Tables:

“DataMyte HandBook Sixth Edition – A practical guide to computerized data collection for Statistical Process Control” by DataMyte Business, Allen-Bradley Company, Inc.

Understanding Gage R&R by Rick Sloop, Quality Magazine, September 2009

BTU Tritan Solar Cell Furnace

Ovens used in the metallization of solar cells can have 20 or more heating and cooling zones.  These ovens can become quite long if the conveyor speed in the initial curing and final cooling sections are the same as the firing section.  By speeding up the conveyor speed in the firing section the oven can be shortened and temperature of the cells can be increased and decreased more quickly.

Multiple conveyor speeds are also encountered in food processing lines where the food product passes through a several machines running at different speeds.

ECD’s newest profiling software can accommodate multiple conveyor speeds.  Most profiling software assumes the

Solar Cell Metalization Time-Temperature Profile

conveyor speed in all zones is the same.  ECD’s new M.O.L.E. MAP® software version 2.18a allows entry of a different conveyor speed for each zone. 

Proper positioning of oven zones on profile time-temperature data requires accurate knowledge of the conveyor speed.  By knowing the sampling interval, length of each zone and the conveyor speed one can calculate how many temperature samples “long” a particular zone occupies.�

1. (Max preheat) How hot the board is just before it hits the wave
2. (Dwell time) how long do you spend in the wave
3. (Contact temp) Temperature of the solder at the contact surface with the board
4. Conveyor speed

All the rest of the many measurable parameters are secondary to these in my opinion. Let’s talk a little about each of these as measured by the WaveRIDER SPC software:

Figure 1: WaveRIDER SPC Software

Max Preheat temp (Preheat Max Temp) is really the temperature of the board (solder joints) just before it hits the wave so you can understand the “shock” the solder joint will get when contact with the wave forces it to melt the solder in a quick few seconds. This temp may NOT be the max preheat temp applied because most wave machines have a large gap between the pre-heater and the solder wave so the board will loose some of the heat during that space/time.

The Dwell time is how long the solder will be above liquidous, all said and done, and this time needs to be consistent across the width of the wave (AKA: parallelism), but you can see consistency across the wave just as easy by setting a min/max spec for the three dwell times. How long you must dwell is a function your solder joint requirements and components ability to withstand the wave temp. Most component specs say they can take 10 seconds or less and most solder joints need only a second to form. The longer you spend above liquidous the more time you give to form the brittle “intermetallic” alloy between the not molten metals. So the only reason you dwell at all is to get the heat to travel up the lead to the board top (on through-hole parts), which may take a few seconds depending on the thermal conductivity of the board and its thickness. A pot of liquid solder has plenty of heat to force into a board, so it depends mostly on the thermal conductivity of the board material itself. You will have to watch/trial your assembly (board) to see just how long that takes so you can set a minimum dwell time. The Dwell time is usually about 2.5 seconds for most 0.062″ (1.6mm) thick boards and this can vary +/- 10% to 15%.

Figure 2: Wave crest pattern

Pot temp at the solder contact surface (Contact temp) does vary depending on the load (your board) places on the surface of the solder wave. It is not the same as the solder pot temp, since that temp is controlled by a Thermocouple deep in the core of the solder pot and not the surface where the board skims across. This temperature should be maintained at least 20ºC or more above liquidous and no more then the maximum temperature allowed by the component specifications.

The conveyor speed is measured by dividing 9.75″ (distance between C and Speed sensors on the WaveRIDER pallet, see Figure 4) by the time measured between the C and Speed sensors hitting the main wave. It is easy to see why this is an important measure since ALL other values will be altered if the conveyor speed changes. Most solder machines can maintain speeds within +/-0.1 ft/minute (+/-1.2 in/minute, +/-3cm/minute)

Immersion depth is only important to prevent solder from spilling over the top of the board during wave contact. The absolute setting is not critical provided it is NOT spilling over the board and its depth is affording the dwell time one needs. However, it is easier to adjust the conveyor speed to control dwell time then the board’s immersion depth and risk a spill over. This is usually controlled by pump RPMs and how full the pot is of solder. If these change over time, the dwell times slowly change and thus is be one of the reasons for dwell changes. However, neither can be directly measured by WaveRIDER which has no chance to access the pump’s RPMs or the amount of solder in the pot.

Conclusion:
Focus your efforts on the critical WaverRIDER measurements by setting reasonable specifications. The many other things the WaveRIDER measures are not as critical, and can be used to diagnose a problem if there is one rather then used to prove there is NO problem. Use the WaveRIDER SPC software to set Spec limits and plot X-Bar R charts for these critical parameters. (Figure 3)

Figure 3: X-Bar R charts

Figure 4: Keep the WaveRIDER pallet contacts clean using a fine wire brush. This will reduce oxide build up on the sensors which can cause the Dwell times vary beyond normal.

Reach For the Sun

Solar Cell manufacturing has been around for a long time; however the materials and process have changed drastically in the past few years and will continue to evolve as the technology and need for Renewable Energy grows. The costs of manufacturing and the risks associated with the ever changing processes can strain  the ability to maintain yields and improve quality. Proving these new manufacturing processes, then achieving the repeatability and yield needed for production have always been a challenge.

ECD’s suite of profiling tools allow detailed Characterization of these advanced processes  during the R&D phase.  During production ramp-up good thermal thermal profile data drives yield improvement leading to maximum profitability.   In  production these same tools can quickly Verify that the optimum manufacturing  process is being maintained.

Thin Film Solar

ECD has many types of customers in the Solar Cell manufacturing industry.  They include start-ups, university research departments and production facilities, located around the globe. 

The application areas that we have been able to identify are among the following:

Silicon Metallization:
   Thermal profiling is used to optimize the drying, rapid firing and following cooling process in the oven.

Silicon Diffusion:
   Thermal profiling is used to optimize the heating, high temperature diffusion and cooling process in the oven.

Thin film Solar Cells:
Done on glass and other substrates, this process is similar to Diffusion process, but at lower temperature.

Profiling Equipment Requirements:
Minimum profiler thickness is important – Many of the ovens designed for solar manufacturing provide little vertical clearance for the profilers.   Thermal barrier requirements vary and in some cases, time and temperature do not allow pass-thru profiling.  In most cases 3 channels of thermal data are sufficient for process verification as all areas of the silicon heat similarly.   Thermocouple attachment can also be tedious, with mechanical pressure being the most common contact method.

We look forward to the continued growth and success of the Solar industry and we would like to invite all solar industry participants to work with us and discover how we can help you reach your performance goals.

Let’s get our terms straight and put some definition to a few key words used here:

Profile – The graphical plot of temperature as a function of time, as measured by one or more thermocouples at points of interest on a PCB

Profiling – Act of gathering the temperature as a function of time data using profiling instruments

Recipe – Oven settings required to produce the desired profile

Zone temperature settings (Set points),

Conveyor speed setting

Convection rate or flow settings

Etc

As a matter of fact, given a specific solder paste, there is only ONE profile you need to worry about-  the one needed by that paste to properly heat the solder paste to make a good metallic bond between components and the substrate (typically plated pads on a PCB), and that does not cause damage to the components. However, depending on the characteristics of each oven (I.e. number of zones, convection rate, conveyor speed, etc) and the thermal nature of the assembly (I.e. its component density, board thickness, etc), there will be different oven recipes needed to make that single profile happen. In other words, different ovens will require different recipes to produce the same profile on a given assembly. Or, different assemblies will require different recipes in a given oven. So if you are soldering many different assemblies in a variety of ovens, you will have many oven recipes on the factory floor, even though you are trying to achieve only ONE profile, this is because you are using the same solder paste on all assemblies.

In the profile graph above, BOTH the Profile and oven Recipe needed to make the profile happen on this assembly are illustrated.

The oven Recipe needed to produce the correct thermal Profile on an assembly is NOT the same thing as a Profile. Or, a Profile is not a Profile, when it is a Recipe. Don’t be fooled when you hear, “We only have three profiles used on our production floor.” Find out exactly what is meant by this.

And remember, Profiling the assembly in the oven in order to fine tune the Recipe which will solder the assembly to meet the Profile needs of the solder paste (and not damage components) is a vital step toward a good thermal quality management program.