Introduction

KIC has introduced a product called the "Manual Virtual Profiler" or MVP™. The MVP is a process monitoring system that allows the electronic assembler to check the profile of the solder reflow process without running a real Printed Circuit Board (PCB).

The MVP is a device that is profiled along with the PCB when the thermal process is first being set up. Then, during production, the MVP can be profiled without need of the PCB to verify the profile. This data is used to calculate the profile of the PCB. We call this calculated profile a "Virtual Profile".

One big advantage of the MVP is its ability to withstand the solder reflow process. A standard PCB is made from FR4 Laminate. FR4 has a "glass transition temperature" that ranges from 120-180C depending on the resin chemistry (http://www.emtworldwide.com/article.aspx?ArticleID=8609). Both leaded and lead-free profiles raise the PCB well above the glass transition temperature which causes the FR4 to dry out and become lighter. After only 5 to 10 profiles a significant change in the thermal properties of the PCB occurs. However, the MVP is designed with materials that have thermal properties that do not change at temperatures below 300C (the typical range of lead free solder reflow processes). Thus it can be profiled over and over without any thermal breakdown.

TC attachment is critical to getting an accurate thermal profile. The results of a profile can vary significantly if the TCs are not attached properly to the PCB (http://kicthermal.com/paper-a-articles/conference-papers/261-a-comparison-of-methods-for-attaching-thermocouples-to-printed-circuit-boards-for-thermal-profiling.html). When verifying the solder reflow profile with the MVP, the operator is not responsible for attaching TCs to the PCB or verifying that the TCs have been attached correctly. Thus a big opportunity for error is removed and the operators responsible for verification profiling do not need as much training as the person that establishes the original profile.

Another advantage is that the same MVP hardware can be moved from oven to oven to check the thermal profiles of all the products in the factory.

The purpose of this paper is to measure the accuracy of the Virtual Profiles calculated by the MVP as they compare to profiles of the actual PCB.

Profiling the Solder Reflow Process

In a typical solder reflow process, the solder paste company lists a solder paste specification which the engineer may then modify or accept as their process window. The process window consists of a number of individual "statistic specifications", and for each statistic there is specified a target value as well as the upper and lower limit. The technician determines a suitable oven recipe and then a "pass-through" thermal profiling device is used to determine whether or not the profile is within spec. Thermocouples will be attached to a sample PCB and then plugged into a pass-through profiler. The sample PCB is often called a "golden board". For a complete description of this process, please read the paper "Finding the Best Oven Recipe with the SlimKIC 2000 Using the Auto-Focus Option". You can find it on the web at: http://www.kicthermal.com/library/Finding%20Best%20with%20SlimKIC%202000%20and%20Auto-Focus.pdf

Once a suitable oven recipe is found and production begins, the technician must periodically verify that the profile is still within spec. Many companies specify that a pass-through verification profile be run at a stated interval, for instance; once a week, once a day, or every time the oven recipe is changed. The more often verification profiles are run, the more expense is incurred. However, allowing the process to drift out of spec between verification profiles could prove even more costly.

The Conventional Verification Process

The conventional means of verifying the process is to run another profile on the Golden Board. However, PCB material (FR4) out-gasses when raised above the glass transition temperature. This out-gassing reduces the weight of the PCB and thus the PCB thermal properties. After about 5 runs the change in thermal properties of the PCB will be noticeable and each additional run takes it further and further from the properties of the target PCB.

Verifying the Thermal Process with the MVP

The MVP is a fixture that can hold a PCB. The oven is first profiled with the MVP and PCB in what is called a "Baseline Profile" (Fig.1). For verification profiles, the PCB is replaced with an adjustable width carrier (Fig. 2).

The MVP fixture contains two of its own thermocouples, one that is attached to a very light material, and a second that is attached to a much more dense material. Both materials are designed to react in the oven similarly to a real PCB, except that they do not experience any thermal breakdown at typical lead-free solder reflow temperatures. The changes in the MVP TCs from the baseline profile (with the PCB) to the verification profile (without the PCB) are calculated and a resulting "Virtual Profile" is displayed. This Virtual Profile is a prediction/simulation of what would have resulted if the actual PCB were profiled, but without having to use the actual PCB.

Fig. 1  PCB installed in the MVP. Configuration for running Baseline Profiles and Verification Profiles

Fig. 2  Carrier installed in the MVP. Configuration for running Virtual Profiles. The carrier width can easily be adjusted allowing Virtual Profiles to be run on multiple ovens for multiple board types with the same MVP and carrier.

First Step:  Measure oven stability and MVP virtual profile accuracy with no changes to the process.

Fig. 3  Screen capture from the KIC 2000 software showing a total of 13 profiles

The list of items on the bottom half of the screen capture in Fig. 3 shows the stored profiles run for this test. The first three profiles are "Baseline Profiles" run with the PCB clamped into the MVP (fig. 1). The next 10 profiles are "Virtual Profiles" using only the MVP with the expandable carrier (fig. 2). All 13 profiles were run in the same oven (Vitronics XPM-3) running the same recipe.

Fig. 4  Process Window specs and limits

The profile and recipe has been optimized for the lead-free process window shown in fig. 4

Fig. 5  Baseline Profile graph and statistic table

Displayed in the graph in Fig. 5 TCs 3-7 were attached to the actual PCB. TCs 1-2 are attached to the MVP and not shown on the graph. KIC uses the "Process Window Index" or PWI to show how well the profile statistics meet the specification. The lower the PWI the better the profile meets the spec. A PWI below 100% is "in spec". The above profile has a PWI of 51% which is well within the specification. For more information on PWI visit http://kicthermal.com/index.php?option=com_content&view=article&id=99&Itemid=99.

Fig. 6  Control charts of profile statistics for 3 Baseline Profiles

As mentioned, 3 profiles were run with the PCB clamped into the MVP. The control charts in Fig. 6 demonstrate the overall stability of the process. The individual chart for peak temperature of the 3 profiles shows the overall peak temperature range is from 233.4 - 241.4C (Fig. 7). The PWI values for this range are 12 - 54%.

Fig. 7  Control chart plotting the Peak Temperature of Baseline profiles

The CpK on the above charts shows that the next profile is very likely to stay in spec.

Fig. 8 Control charts of profile statistics for 10 MVP virtual profiles

We then ran 10 MVP virtual profiles. Above are the SPC charts for these 10 profiles (Fig. 8). We expect that each of these Virtual Profiles would closely predict the profile of the actual PCB. The overall PWI range is from 54 - 58%. The peak temperature range is 233.8 - 2.41.7C (Fig. 9).

Fig. 9  Control chart plotting the Peak Temperature of virtual profiles

The 13 profiles (3 with the PCB and 10 with the MVP) were run in a brand new Vitronics XMP-3 forced convection reflow oven running the same oven recipe. The calculated CpK shows that this process is extremely stable.

This first set of profiles shows that the MVP can accurately predict the process thermal profile when no changes occur. The next step will be to show that the MVP can accurately predict the thermal profile when the oven temperature or conveyor speed is changed.

Second Step:  Measure MVP virtual profile accuracy for scenarios where process changes are occurring.

After running the previous 13 profiles through the same oven recipe, we decided to change the oven to determine two things:

1. Does profiling the oven with the MVP instead of the actual PCB detect changes to the oven?

2. How closely does the resulting Virtual Profile match the actual PCB profile?

Changes to the oven:  4 scenarios

We changed the oven recipe in four different ways to try to mimic a typical field failure:

• Increase the conveyor speed by 10%
• Decrease the conveyor speed by 10%
• Decrease zone 7 by 15C
• Increase zone 8 by 15C

For each of these scenarios 4 profiles were run:

• 2 Virtual Profiles were run with the MVP and carrier
• 2 PCB profiles were run with the PCB in the MVP to measure the actual changes to the PCB profile

This allows a comparison to be made between the Virtual Profile (which was calculated using the MVP data) and the actual PCB profile.

Increase Conveyor Speed by 10%

The last dot in the above charts shows the virtual profile with the conveyor speed increased by 10%. Notice that while the PWI is still in spec, the CpK is now indicating that this process could go out of spec soon.

Above is the Virtual Profile. Notice the PWI is predicted to increase from 50% to 78%. The peak temperature is predicted to decrease, which is exactly what we expect from an increase in conveyor speed.

Decrease Conveyor Speed by 10%

The last dot in the above charts shows the virtual profile with the conveyor speed decreased by 10%.

Above is the Virtual Profile. Notice the PWI is predicted to increase from 50% to 102%. The peak temperature is predicted to increase, which is exactly what we expect from a decrease in conveyor speed. The total time above 217C is going to change the most and is the statistic that will push this profile out of spec.

Decrease Zone 7 by 15C

The last dot in the above charts shows the virtual profile with Zone 7 setpoint temperature decreased by 15C. Again, even though the PWI is still in spec, the CpK is in alarm indicating that this process is likely to be out of spec soon.

Above is the virtual profile. The Peak Temp has dropped 3 - 4C which is not surprising given that zone 7 has been lowered by 15C.

Increase Zone 8 by 15C

The last dot in the above charts shows the virtual profile with zone 8 setpoint temperature increased by 15C. This is a major problem as the predicted PWI is 140%. A runaway zone 8 is not something we can live with, even temporarily.

Above is the virtual profile. The Peak Temp is predicted to increase by as much as 10C.

Measuring the Accuracy of the Virtual Profile

The previous four pages demonstrate that the MVP is capable of detecting changes to the oven recipe without having to profile the proces with the actual PCB. Now we need to show that the changes predicted by the MVP are accurate. To do this we will create four separate bar charts, one for each Process Window statistic. The first statistic is "Average Maximum Rising Slope":

Bar Chart 1:  Average Maximum Rising Slope

There are five pairs of bars on the above bar chart. Each pair has the same color. The first 2 bars show the Average Maximum Rising Slope for two baseline profiles. The other bars also show the Average Max Rising Slope, but for the four modified recipes:

• Red - Conveyor Speed increased by 10%
• Light Blue - Conveyor Speed decreased by 10%
• Blue - Zone 7 decreased by 15C
• Orange - Zone 8 increased by 15C

The first red bar indicates that the VP predicted a slight increase in Max Rising Slope. The second red bar shows that the actual Max Rising Slope did increase, but not quite as much as predicted. Our Process Window (page 3) allows the Max Rising Slope to vary between 0 and 3C / second. We can see that none of the recipe changes we tested caused the Max Rising Slope to change significantly. 

The same color coding scheme is used on the bar charts for the other three process window statistics shown on the following pages.

Bar Chart 2:  Soak Time 140 - 160C

The statistic "Soak Time 140-160C" is a measure of how many seconds the thermal profile spends on its way up from 140C to 160C. During the Baseline Profiles the average soak time was about 62 seconds. Increasing the conveyor speed by 10% dropped the soak time significantly. The VP predicted it would drop to 53 seconds. The actual was 54 seconds (red bars). Decreasing the conveyor speed increased the soak time significantly. The VP predicted a rise to 74 seconds, the actual increase was 71 seconds (light blue bars). The prediction is not perfect, but still extremely useful. 

Below are the same bar charts for the "Peak Temperature" and "Time Above 217C".

Bar Chart 3:  Peak Temperature



Bar Chart 4:  Total time above 217C

The above bar charts show that, while the Virtual Profile is not perfect in predicting profile statistics, it is clear that the MVP with its expandable carrier can be used to verify product profiles.

Advantages of the MVP

There are many advantages of profiling with the MVP instead of the actual PCB to verify your thermal process:

• PCB Breakdown - Profiling the same PCB over and over will literally dry it out and change its thermal characteristics. After as few as 10 runs at typical solder reflow temperatures the PCB will behave differently from your production boards. But the MVP is designed with materials that do not breakdown at temperatures up 300C.
• Thermocouples Attachment Responsibility - Knowing where and how to attach thermocouples is a big responsibility. However, once the Baseline Profile is complete, a lower qualified technician can run the verification profiles with the MVP without having to be qualified to recognize whether or not the TCs are still attached correctly.
• Minimize Clutter and Confusion - No need to keep shelves fuill of PCBs used for verification profiles. The same MVP and carrier is used to run verification profiles on all your lines and all your produts.

Conclusion

KIC's Manual Virtual Profiler (MVP) is a process monitoring system that allows the electronic assembler to check the profile of the solder reflow process without wearing out a real PCB. The MVP makes verification profiling quick, each, and practically fool-proof. It will significantly reduce the cost of verifying your thermal processes.