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Overview Of Temperature Monitoring In Forced Air Convection Ovens

Industry Overview Series: #00100 Rev: 06/01/95

Introduction
The Details of Temperature Monitoring
Common Problems and Solutions
1. PROBLEM: The oven shows that all the zones are being controlled to the setpoint temperature, but reflow is not occurring properly.
2. PROBLEM: A lot of PCBs has reflowed properly. A period of time later, a similar lot fails to reflow properly or overheats.
3. PROBLEM: Even though the oven controller is showing that the setpoints and conveyor speed are correct, and the KICprobes are showing that the temperature along the oven conveyor is within tolerance, the PCBs still do not reflow correctly.
4. PROBLEM: A PCB fails at test or in the field weeks or months after it has been assembled.

I. Introduction

The reason for part and oven profiling is to improve yield by reducing production defects such as solder bridging, cold solder joints, thermal shock related part failures, solder joint fracture due to intermetallic formation and board warpage. A part profile is a graphical view of the actual temperature of the part as it moves through the oven. This profile is used to set up and fine-tune the desired oven temperature for each zone of the oven. Profiling a forced air convection oven will be easier than profiling an oven that uses IR bulbs or IR panels. This is because compared to other types of reflow ovens, Forced Air Convection Ovens provide more efficient heating.

II. The Details of Temperature Monitoring

In order to ensure the highest possible first pass yield and least amount of rework related costs, it is important to monitor both the oven process and the product profile. Separate tools are used to completely fulfill these monitoring requirements. The first tool is the KIC Prophet Thermal Manager(tm) system, which consists of WinKIC(tm) Software and two 1/4" diameter tubes ("KICprobes") that each contain 15 "stationary" thermocouples (TCs). These KICprobes run the length of the oven and are mounted above or below the oven conveyor as close to the path of the PCBs[1] as possible. They are designed to monitor the temperature along the oven conveyor (see Figure 1). In a rail conveyor oven, the KICprobes are often mounted to the rail with supplied brackets. KICprobes are only 1/4" in diameter and do not interfere with standard oven operation.

**Figure 1. Oven Setup**
Units of Measurement Inches Tunnel Start Tunnel Start Load Table Length 30.00 to to Tunnel Length 166.5 Zone Zone Start Control TC Unload Table Length 30.0 1 4.2 11.0 2 17.7 24.5 Load Table Start to KIC Oven Start 30.0 3 31.2 38.0 KIC Oven Start to KIC Oven End 166.5 4 44.7 51.7 5 58.7 68.2 Probe Feed Point ENTRANCE 6 77.7 84.6 KIC Oven Start to first KIC TC 0.00 7 91.5 98.2 Total Number of KIC TCs 30 8 105.0 111.8 KIC TC Spacing 5.50 9 118.7 125.6 Oven Number 1 10 132.5 138.5 Distance to End of Last Zone 144.5 Number of Zones 10 Zones Setpoints Top & Bottom No Oven Type Solder Reflow Heat Source Forced Air Convection

Figure 1. Oven Setup


 Units of Measurement               Inches                                     
                                                  Tunnel Start     Tunnel Start
 Load Table Length                   30.00              to              to     
 Tunnel Length                      166.5   Zone   Zone Start       Control TC 
 Unload Table Length                 30.0      1        4.2               11.0 
                                               2       17.7               24.5 
 Load Table Start to KIC Oven Start  30.0      3       31.2               38.0 
 KIC Oven Start to KIC Oven End     166.5      4       44.7               51.7 
                                               5       58.7               68.2 
 Probe Feed Point                 ENTRANCE     6       77.7               84.6 
 KIC Oven Start to first KIC TC       0.00     7       91.5               98.2 
 Total Number of KIC TCs             30        8      105.0              111.8 
 KIC TC Spacing                       5.50     9      118.7              125.6 
 Oven Number                          1       10      132.5              138.5 
                                            Distance to End of Last Zone 144.5 
 Number of Zones                     10                                        
 Zones Setpoints Top & Bottom        No                                        
 Oven Type                      Solder Reflow                                  
 Heat Source                    Forced Air Convection

The KICprobes plug into either "KICboards" or into the KIC "Satellite". KICboards are cards that install in any AT compatible PC (one per KICprobe), and the Satellite is a self-contained data acquisition system that mounts near the tunnel entrance and connects to the PC via COM 1, 2, 3, or 4.

The KICprobes can be used to determine exactly when the oven temperature along the conveyor has stabilized. Even ovens using forced air convection heating will sometimes indicate that they are ready 5 to 10 minutes before they are actually stable.

**Figure 2. Product Setup**

To monitor or "profile" a product, a second type of tool is used. This can be either a KIC TC Extension or the SlimKIC Thermal Profiler(tm) wireless remote Thermal Transmitter. To monitor the temperature of a PCB, up to 12 "traveling" TCs are soldered to a test PCB using 10-88-2 solder (eutectic = 268°C). The placement of the TCs on the PCB is critical and this information is entered into the software via the "Setup/Product" command (see Figure 2).

As the leading edge of the PCB enters the oven, the user presses the Start Button. This tells the WinKIC software to begin recording PCB temperature.

When the leading edge of the PCB exits the oven, the Stop Button is pressed, and the WinKIC software calculates the actual conveyor speed. If the conveyor speed is off by more than 1%, a warning is issued (see Figure 3).

**Figure 3. Belt Speed Warning**

If the first pass of the PCB thermal profile is not correct, the Software can be used to create a mathematical model of the oven environment. With this model the user can enter setpoint and/or conveyor speed changes and the KIC System will instantly display an accurate prediction of the resulting PCB profile. This "Profile Prediction" capability will usually allow the user to set up the oven by running the test PCB no more than 2 or 3 times. This is a considerable time savings when compared to the alternative of 5-10 passes when not using profile prediction.

Once the necessary oven setpoints and conveyor speed have been established for a given PCB (the oven is "profiled"), the next step is to establish an acceptable variation in temperature along the oven conveyor. In each zone of a typical forced air convection solder reflow oven, air travels through a fan, past a heating element, past a control TC, and then down to the conveyor (see Figure 4). The power to the heating element is regulated according to the temperature of the control TC and the speed of the air. The faster the air is moving, the closer the temperature at the conveyor will be to the control thermocouple temperature.

**Figure 4. Typical Forced Air Convection Zone**
	The fan blows air through the heating element. The air then passes over the control TC and down to the conveyor. Some portion of the air is recirculated to the fan. The oven controller regulates power to the heating element to maintain the control TC at a constant temperature. The temperature at the oven conveyor is dependent upon the control TC temperature and the air speed. The faster the air is moving, the closer the temperature at the conveyor will be to the temperature at the control TC.

**Figure 5. Graph**

The air speed is extremely critical in forced air convection ovens. However, at the time of this writing, no Forced Air Convection Solder Reflow oven offered the ability to monitor the air speed in each zone. By using the KIC Prophet to monitor the air temperature at the conveyor, and using the oven controller to monitor the temperature at the zone control TC, the effective air speed can be inferred. That is to say that for a given zone setpoint temperature, the air speed cannot be changed without also changing the temperature at the conveyor. The temperature at the conveyor is recorded every 15 seconds, 24 hours a day, every day. WinKIC Software automatically saves this information permanently and provides easy access to the data at any time (see Figure 5).

Virtual Profiling

The WinKIC Prophet system has a feature called "Virtual Profiling" that provides an easy way to determine how temperature changes along the conveyor during production affect the PCB thermal profile. Virtual Profiling works like this:

Use the Prophet system KICprobes to determine that the oven has stabilized.
Attach thermocouples to a test PCB and use the SlimKIC thermal transmitter to record the PCB thermal profile.
Click on the WinKIC Prediction/Virtual Profiling "Crystal Ball" to create a mathematical function that relates the KICprobe temperatures to the PCB thermal profile. Save this relationship in a WinKIC "Process" file.
Load the WinKIC Process file and enable Virtual Profiling. Every 15 seconds, as the KICprobe temperatures are updated, a Virtual profile for the PCB will be calculated and displayed.

Virtual Profiling allows you to see how loading, air flow, nitrogen flow, etc. affect the thermal profile of the production PCBs. "Warning" and "Error" bands can be placed around the Virtual Profile. When the profile exceeds the warning band, the Virtual Profiling Crystal Ball turns yellow. If the profile exceeds the error band, the Crystal Ball turns red and an alarm light can be activated (see Figure 6).

**Figure 6. Virtual Profile**

Along with the Virtual Profile, the KICprobe "Average Deviation" is displayed. This number is the average number of degrees that the current KICprobe temperatures are deviating from the target KICprobe temperatures (see Formula 1). This gives a clear indication of how much the oven temperature has changed since the last time the PCB thermal profile was measured.

Limits can be set around the average deviation to indicate a warning or error situation and an alarm light can be activated.

Formula 1.  Average Deviation
Average Deviation = 
i=30
 
i=1
KICprobe Current{i} - KICprobe Target{i}

**Formula 1. Average Deviation**
Average Deviation =	i=30 i=1	KICprobe Current{i} - KICprobe Target{i}

III. Common Problems and Solutions

1. PROBLEM: The oven shows that all the zones are being controlled to the setpoint temperature, but reflow is not occurring properly. This is typically because the airflow within the oven has changed.

The oven does not have the ability to alarm the user if the air flow is restricted or increased. In fact, changing air flow will have no effect on the temperatures displayed by the oven controller. This problem can occur when:
CAUSE:

Someone inadvertently adjusted a flow meter.
PCBs are placed too close together.
A fan in the oven is weak or broken, or a flow meter is not regulating correctly.

SOLUTION: To prevent this from reoccurring, you can use the information gathered from the KICprobes which is stored automatically in your PC. Using the WinKIC software History Files, you can determine the exact time the oven began to falter, and the first place within the oven where the trouble occurred. This should help pinpoint the problem.

2. PROBLEM: A lot of PCBs has reflowed properly. A period of time later, a similar lot fails to reflow properly or overheats.

CAUSE: This can be caused if the spacing between the PCBs on the belt has been changed.
SOLUTION: To prevent this from occurring, use the WinKIC Virtual Profiling feature to sound an alarm. As each PCB goes through the oven, it removes a certain amount of heat from each zone. It takes some time for the temperature along the oven conveyor to recover. Using the Virtual Profiling feature, any change in the product profile would be known instantly, the moment it starts to deviate. If the PCBs are placed too close together on the conveyor, the next PCB will reach the zone before the zone temperature has recovered. Virtual Profiling will indicate this immediately with a warning and if not corrected, it will trigger an alarm.

3. PROBLEM: Even though the oven controller is showing that the setpoints and conveyor speed are correct, and the KICprobes are showing that the temperature along the oven conveyor is within tolerance, the PCBs still do not reflow correctly.

CAUSE: This could be caused by the oven conveyor speed being out of calibration.
SOLUTION: In this case, you would go back and check the thermal profile using a pass through profiler. A belt speed calibration error will be clearly indicated. (Refer to Figure 3)

4. PROBLEM: A PCB fails at test or in the field weeks or months after it has been assembled. This could have been caused by one, or a combination of several things including, but not limited to:

CAUSE:

Defective components or component leads.
Defective solder paste or other material of attachment.
Improper or inaccurate deposition of solder paste.
Air entrapment in solder paste.
Solder paste/flux separation during deposition.
Damage to the PCB during cleaning, shipping, or in the field.
Faulty solder reflow or a problem with the reflow oven itself.

SOLUTION: Conduct a systematic review to find the cause. If a component is defective, refer back to the manufacturer. To check for the solderability of component leads, perform a solder dip test as specified by IPC guidelines, and also refer back to the traveler that should be included with each lot of components purchased. Items b) through e) can be visually inspected, microscopically as needed, to determine faults in material, material dispensing or from other types of physical damage.
If the fault lies in the solder reflow area, it is easy to use the WinKIC History Files to backtrack and see what exact temperatures the PCB experienced during reflow. The WinKIC software automatically logs KICprobe temperatures every 15 seconds. If you know the precise date and time the PCB went through the oven, you can determine the oven temperatures that the PCB was exposed to and whether the oven was working properly.
To help prevent this in the future, use the Virtual Profiling feature of the WinKIC software to monitor the profiles of all production PCBs as they are run.
By establishing a system of maintaining "travelers" for each lot of boards (one that stays in a computer log file, and one that is printed and stays with the actual boards) at the time they are completed, checking on the reflow status of any particular board is an easy matter.

[1] The term "PCB" (printed circuit board) is used in this text as an example. All information in this note is equally applicable to any type of board, device or product that is processed using a Forced Air Convection oven.

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