Energy needs a conductor in order to flow

1) An energy difference between two objects. 

and 

2) There is a conductor to act as a bridge enabling the energy to flow. 

Energy always flows through a conductor from an object of high energy to an object of low energy. In this illustration, the high-energy object is a moving hammer, the low energy object is the table and the conductor is a block sitting on the table. 

When you hit the block with the hammer, the energy contained in the moving hammer is transferred to the block when it hits. Some is also conducted through the block and transferred to the table it is sitting on. However, because the block is not a perfect conductor, which is true for most things, some of the energy stays in block. That energy bounces between the molecules of the block like balls on a pool table. 

Because the molecules rub up against each other, and there is friction between them, some of the moving energy of the hammer is converted to heat energy, which causes a rise in the block’s temperature. It all comes down to molecular motion in an imperfect conductor creating friction that raises its temperature. Therefore, temperature increase is a way of observing energy flow, and energy flow that causes a temperature rise is called heat flow. 

What Affects Heat Flow? 

Heat "flows" from hot to cold

The amount of heat that flows is dependent on the same basic things as energy flow, only we sense its effects by measuring temperature difference. As with high-energy objects imparting their energy to low-energy objects through a conductor, heat flow happens when a hot object transfers its heat through a conductor to a cold object. The quality of the conductor determines how quickly, and how much of the heat is transferred. If no heat is lost to the outside, the objects and the conductor in the above illustration would settle to a temperature 1/2 the value of their difference above the cold object, or below the hot object. The quality of the conductor would then only affect the time it takes for the heat flow to even out the two temperatures. 

Heat flow, therefore, is affected by the temperature difference between the Hot and Cold object and the quality of the conductor. The higher the temperature difference, the faster the heat flow and the higher the conductivity of the conductor, the faster the heat flow. 

Methods of improving this conductor have been played with by oven manufactures for many years. Three common ways to conduct heat between the oven heater (the heat source) and your circuit boards (the heat sink) are: 

1) Infrared Radiation (IR) 

IR depends mainly on the “sun-like” conduction of heat via electromagnetic radiation. Although some conduction occurs due to heating of the atmosphere surrounding the circuit board, it plays a lesser role than the radiant energy. This is useful when the entire surface being heated will absorb the radiant energy in exactly the same way at exactly the same rate. The main drawback is that in order to make up for the inefficiencies of this method of heat transfer, the heat source is often set to temperatures much higher than the required melting temperature for the solder on your circuit board. In addition, the components on your circuit board each absorb radiant energy at different rates, aka emissivity. While one component may heat very little, another may be over-heated because it absorbs radiant energy very well. This uneven heat absorption makes it very difficult to choose a heat-source temperature that will meet the varying absorption rates of the variety of components on a typical circuit board. 

2) Vapor Phase (hot liquid) 

Vapor phase heat conduction is one of the best around because of its high conductivity and heat capacity, which is released as the vapor changes from gas (vapor) to liquid. Plus, it has the ability to conform to the irregular shape of your circuit board. This method of conducting heat in reflow soldering machines is currently enjoying a comeback. 

3) Convection (hot air) 

Convection has been very popular because it provides “vapor like” conformance to board shape, but does not require much more than electric power to function. Here hot air (or nitrogen) is used as the conductor, but with an extra kick: convection fans that move heated air from the heat source to the heat sink. This combination of an air conductor and air movement is called convection. The greatest asset of convection is that it allows you to lower the temperature of the heat source because the movement of the air creates a more efficient heat flow system. 

4) Direct conduction (metal plate) 

Using a direct conductive medium, like a metal plate, is efficient, but is only practical when the surface being heated is flat. 

What about my boards? 

In a convection oven, heat is "hammered" into your circuit board by moving air

So heat flow causes circuit boards to get hot within an oven. In this case, the oven is the heat source (like our hammers); moving air is the conductor that delivers the energized air (like moving the hammers) and pounds the heat into the circuit boards (the heat sink).

Because of the increased conductivity that the moving air creates, lower oven temperatures still allow a circuit board to reach the required temperature, and more evenly because conducted heat is not as dependent on a component’s ability to absorb radiant energy. The heat is conducted to the circuit board as fast as the moving air can deliver it. 

So, if you want to increase the heat flow, you just turn up the temperature, right? That is correct, but this creates a potential hazard for your circuit boards. Given enough time, your circuit boards will reach the heat source’s temperature. If that temperature is significantly higher than the specified limits for your components, you will damage them. Another way to increase the heat flow is to set the temperature at an acceptable level and only increase the airflow. In other words, add more hammers. This forces the heat into your circuit board components much more evenly and at a rate that meets your component specs. You can accomplish this at your desired process speed while lowering the risk of component damage due to over-temperature. 

How can I measure heat flow?

  

Heat flow is easy to understand, but difficult to measure. This is because the conductor of the heat flow (air) plays as big a role as the temperature in determining how much heat actually flows. For heat, everything is a conductor. There is no simple way to say where heat is going; it goes everywhere. So you cannot know how much of the energy consumed by your heater actually gets to your circuit boards, even though heaters are nearly 100% efficient at converting electrical energy to heat. Therefore, energy into an oven is not energy transferred to your circuit boards, because a lot of it is going into the room, up the exhaust, and some into you.

As a result, heat flow measurement has taken the form of very sensitive probes that measure the temperature difference between the surfaces of thin films. These measurements translate into the amount of energy in watts per square centimeter. Typical values for heat flow in a reflow solder machine from the top heating zones to the surface of a circuit board range from 0.1 to 0.7 watts/sq. cm. This value drops as it proceeds through the oven because circuit boards heat up and reduce the temperature difference between the heater and the circuit boards. This naturally reduces the heat flow because heat flows from hot to cold and if there is no more cold, there is no place for the heat to flow – no heat flow. 

Problem Solved 

Convection plays an important role in heat flow

Understanding that oven temperature alone “does not a temperature profile make” is vital toward making your oven consistently reproduce a good temperature profile. The temperature profile your circuit board experiences as a result of your oven settings is dependent on the temperature settings AND the heat flow capacity of your oven. To illustrate this fact, two thermal profiles were run on the same circuit board where the speed of convection air in the oven was the only parameter changed. As you can see, the thermal profile at low convection is dramatically lower then the same profile at high convection, while the oven’s zone temperature settings remained the same.

This does not mean that the heat flow capacity should be large, but it must be consistent hour-to-hour, day-to-day, and week-to-week. Small variations in heat flow can cause temperature variation on your circuit board, even though the oven temperature remains the same. A measuring device that can produce meaningful information relative to heat flow in addition to the temperature will give you a complete understanding of your oven and its performance over time. Of course I would not tell you this if I did not know of the tool that can measure your oven’s ability to “hammer heat” into your boards; that being the OvenRIDER. 

Conclusion

 

Controlling the reflow oven soldering process requires monitoring not only temperature but also heat flow. Since heat flow is what causes objects to increase in temperature, measuring your oven’s ability to increase the temperature of fixed thermal mass objects will allow you to detect variation in heat flow. By observing the temperature rise in the mass over a fixed amount of time, you can discover if the same amount of heat has been transferred into the mass. Such a measurement, combined with ambient temperature, yields a much more complete profile of a reflow oven. Only by knowing the heat flow capacity of an oven can you be assured that the oven will heat your circuit boards at the same rate and to the same temperature, every time.

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.

A good Thermal Quality Program (ThQM) demands consistent oven verification to show that the reflow oven is reproducing the same thermal environment as it has in the past. The OvenRIDER is a good way to verify oven performance. How do you make best use of these tools, both the OvenRIDER pallet and the OvenRIDER SPC (ORSPC) software?

There are five basic steps:

• Set up the Oven, Workbook and MOLE
  
• Model the oven
  
• Collect a base set of OvenRIDER runs
  
• Set the spec limits
  
• Monitor of verification

Let’s get right into it:

Set up the Oven, Workbook and MOLE

1. Place a magnet at the beginning of zone one. The OvenRIDER came with at least three magnets. This is not because we think you only have three zones, but because we figure you may have more then one oven, and you only need one magnet per oven to mark the start of zone one and let the OvenRIDER measure conveyor speed for you. The software detects this magnet in the profile and uses it automatically place the start of the oven model on the profile’s time axis. It does not matter where you put this magnet as long as it is on or before the beginning of zone one. You can even put it outside of the oven so it can be removed when you are not using OvenRIDER. Just be sure you always put it back in the exact same spot.
2. Set the oven’s recipe you plan to use for this OvenRIDER run. It can be the same recipe you are now using to solder boards. In fact, you may want to start an OvenRIDER collection of runs for all your different recipes. This does need to be a big one time effort for all your recipes. You can do this slowly for each recipe one at a time as you do line change over. In time you will have enough runs for each recipe to begin useful oven Verification.



3. Start the OvenRIDER software and select the Workbook you wish to save these recipe runs. If this is the first time you opened the software, a sample work book will be opened. Go to the file menu and close this Workbook. Then open the file menu and start a New Workbook or open an existing Workbook you previously created. The Workbooks can help you group like OvenRIDER runs. Having a work book for each oven recipe is the best for Oven Verification. You can name Workbook the same as the recipe name to help keep it straight.�
    
4. To make sure the MOLE is ready, connect it to the PC via the communication cable and run the Configuration Wizard in the OvenRIDER menu. This will find the MOLE and set it’s clock and other settings.

 



 

5. Load the MOLE into the OvenRIDER barrier and connect the configuration plug

 



 

6. Make sure the oven is up to temperature and ready to receive product. Also make sure the conveyor width is set the OvenRIDER’s width.
7. Start the MOLE, close and lock the lid, and load the OvenRIDER pallet onto the oven conveyor.
8. When it gets through the oven, open the barrier and stop the MOLE by pressing and HOLDING the button until the Status LED turns off by itself.

Model the oven

Oven modeling is critical. This tells the software how big each oven zone is and where each zone is on the thermal profile so it can correctly show which zone had what effect on that portion of the profile.

Above is a typical Oven RIDER thermal profile with an oven model (vertical dashed lines) in place. Each zone influenced the “shape” of the thermal profile of the OvenRIDER’s sensors as it passed through oven. The amount of influence depends on the oven recipe temperature set point for each zone, the conveyor speed and the convection rate (fan or air speed). The temperature set points and conveyor speed has a straight forward and expected effect. The convection rate is a little different. Some ovens allow this to be changed and may be measured in several ways like: velocity, pressure, percent, Hz or RPMs. In some ovens, the convection rate is not adjustable or has two or three settings like: Low, Medium, and High. In either case, convection rate changes are one of the leading causes of profile changes when neither the conveyor speed nor zone temperature set points have been altered. So understanding what part of the profile each zone influenced is critical to pin pointing where problems in the oven have occurred.

Here is the best way to model an oven, any oven:

1. After the profile run, connect the MOLE to the PC via the communication cable and press the Read OvenRIDER button.

 

2. The software will as you how much data you want to use from the previous run, if there is a previous run. This time, the first time don’t select anything by un-checking all three.



3. The OvenRIDER profile will look something like the profile below. Because you installed a magnet, the start of zone 1 is in place on the profile, and that’s it. It is ready to Model the oven.

 

4. Start the Manual Zone Definition by selecting it on the Manual Zone menu.
5. Select the number of zones check boxes on the let that your oven has. Name them if you like. Leave the values it defaults for Zone Lengths and Units. We will take care of that next. Note it has the measured conveyor speed already calculated and entered in as the conveyor speed value.



6. Enter the Zone Temperature Set Points in the Top column. Note the Bottom values copy from the Top values as you enter them since most oven use the same Top and Bottom values. Enter the bottom values, if different, after you enter the Top values. When done, click OK.
7. You will now see all of your zone boundaries located in the default locations, but not in the right locations. Note the small boxes at the top of each boundary. This is a handle for you to grab and move the boundaries to the right locations. The right locations are in that little dip that happens between zones you can see in the Ambient sensors, the Red, Blue, and Green profile lines (Channels 1, 2, and 3). So start moving the zone boundaries into place. Remember, don’t move the start of the first zone, this was set for you by the software when it detected of the magnet.



8. Your profile and oven model should look like mine below. This is the “thermal” Model of your oven. It’s already saved with this profile, but let’s save it for future use with other recipes used in this same oven.



9. Go to the Manual Zones menu and select Setup Zones.



10. This will re-open the Oven Zone Setup dialog box. Hit the save button and name your oven model. I recommend using the Oven or line name. This model can be used for other OvenRIDER profile runs using the same model of oven, but with different oven recipes later on. Use the Load button to recall this oven model on future runs.

Collect a base set of OvenRIDER runs

You now have a good oven model and your first run at this recipe. You need to collect at least three runs at this same recipe before enough data is collected to meaningful, statistically. This can be all at once, but this take a big chunk of time from production, and most don’t have that kind of time. I recommend you take an OvenRIDER run at the beginning of each day or shift, while you are running the same recipe, for several days. You will have 3 or more runs in less then a week.

If you don’t run the same recipe for more then a day, you can set an “Oven RIDER Recipe” near the recipe setting you use most often. Start the day with that recipe, take the run, and then move to the “real” recipe you plan to use that day.

If you want to do three or more Oven RIDER runs all at once, just be sure to cool the MOLE and the Pallet to room temperature between runs. Do NOT short cut this critical cooling process. This will save the MOLE from possible over heating and assure the pallet temperatures are consistent run to run. 20 minutes on a nice fan will be enough in most situations.

Once you have several runs, you Spreadsheet will look something like this:

Make use of the user columns, the Green ones. This is where your can name the column anything you like. At least make a column for Recipe, and maybe the Machine or line name. I also have a Part Number column to identify the part or assembly soldered using this recipe.

Set the spec limits

With a good set of OvenRIDER runs collected under the same oven recipe, at least three, but five or even 10 is better, you can now set limits on the average of these runs. Do the following:

1. Select the Spreadsheet tab and filter out or “hide” the runs that are NOT part of the base OvenRIDER runs. If you named each base run like I did in the example, this is easy. Select the filter dropdown arrow in row 4 of the column where you named the base runs, n my case column E, and select the name that you used to name the base runs. I used “Base Runs,” for simplicity. This hides (temporally) all the other runs. If you don’t have any other runs yet, then you don’t need to do this. The statistical data at the bottom of each column is now calculated only from those runs still visible.



2. Now select the Admin tab and click the drop down arrows in the LSL and USL columns. Select the -10% and + 10%. This will set as upper and lower spec limits +/- 10% of the Average values for your base line runs showing on the Spreadsheet tab for ALL the values OvenRIDER measures. These will be good starting values to see how your oven is doing.





3. Optional – You may want to select +/- 5% later on after you have a few more runs, if you wish to run tighter specs. Or, you may want to change the conveyor speed spec limits to a little tighter because you know you machine can do better then 5% or 10%. Finally you may want to remove some of the specs because you are not concerned with those measured values. To start with, it will not hurt to keep all the values.

Monitor of verification

Now each time you run the OvenRIDER at this recipe, you can see in a flash if the ALL of the measured values are within the specs you set. Simply select the OvenRIDERData tab and give the values a look.

If any of them are Red (above spec) or Blue (below spec), then there may be a problem in this portion of the oven. The values of most concern are the Average Temperatures and Process Delta per zone. If these begin to fall out of spec, then you know there may be a problem in that zones ability to heat.

All black, as in this run, the oven is ready to roll.

Any parameter out of spec should be examined to see if it’s a concern or not.

If you wish to get deeper in the SPC control you can create. Xbar-R Charts for any of these values as well. Simply drag and drop any measured value in the column to the left into one of the SPCA, B, C… boxes. Check the box to add that SPC group of charts to the tabs pages below.

Select the tab to see the X-bar R charts. Here are many production runs of the oven ruder captured of time.
These chart use all the typical SPC rules to determine process control.