Profile Parameters for Reflow and Wavesolder Processes
Application Note #00121 Rev: 98-06
- Application Description
- The SlimKIC Thermal Profiler
- Data Logger
- Direct Connect
- SlimKIC Setup
- Solder Reflow Process
- Peak Temperature
- Maximum Slope (Ramp Rate)
- Time Above Reflow
- Wave Solder Process
- Common Problems and Solutions
- LIST OF FIGURES
- Figure 1 : Illustration of “SlimKIC Transmitter Set-up”
- Figure 2 : SlimKIC data logger set-up with Direct Connect Cable
- Figure 3 : Typical Reflow Oven Setup
- Figure 4 : 9-channel TC Yoke with thermocouples
- Figure 4a : TC Harness
- Figure 5 : KIC Thermocouple Position Map
- Figure 6 : The KIC Software automatically calculates key profile “statistics”
- Figure 7 : Belt Speed Warning screen shot
- Figure 8 : SlimKIC Profile Prediction
- Figure 9 : Auto-Predict screen shot
Thermal profiling is the process of plotting product temperature over time during a thermal process. A product’s “thermal profile” is determined by initial product temperature, oven temperature, conveyor speed, and the heat transfer rate between the oven and the product. This Application Note assumes you have a working knowledge of thermal profiling. For an introduction to thermal profiling, check out “The Science Behind Conveyor Oven Thermal Profiling”.
The SlimKIC(tm) Thermal Profiler is a device designed to easily and accurately record the thermal profile of Printed Circuit Boards (PCBs) and other products in a variety of thermal processes. The SlimKIC Thermal Profiler can be used as a real-time thermal data transmitter, a direct-connect profiler, or as a data storage profiler (Data Logger). The SlimKIC Software is available in Microsoft® Windows(tm) and can monitor the temperatures of up to 12 thermocouples.
The SlimKIC is a wireless remote thermal transmitter that follows behind the PCB on the conveyor. It transmits temperature data from up to 12 TCs attached to the PCB via radio frequency to a remote receiver located outside of the oven (See Figure 1). The SlimKIC Thermal Shield allows the transmitter to operate under a wide range of temperatures and to be easily handled without gloves immediately after exiting the oven.
The SlimKIC is used in profiling PCBs for both solder reflow ovens and wave solder machines. It can also be used to monitor solder pot dwell time. The SlimKIC receiver plugs into the RS-232 port of your PC, and is available with KIC’s proprietary thermal processing software in Microsoft® Windows(tm). KIC software is continually upgraded, and all upgrades are free to SlimKIC users.
The SlimKIC has the capability to store and download the information to a PC after the profile is completed. This makes it possible to profile without having a computer on the manufacturing floor. To use the SlimKIC in this fashion, a direct connect cable is connected to both the PC COM port and the SlimKIC. The KIC software is used to set up the storage time and number of TCs to be used. These instructions are downloaded into the SlimKIC, and it is disconnected from the PC, then taken to the process to be profiled. Once the TC’s are attached, a flip of the start switch begins the profiling process. The SlimKIC will store data for the pre-programmed time. Once the profile is finished, the SlimKIC is again hooked up to the PC with the direct connect cable, and the thermal data is transferred from the profiler into the computer.
Even if purchased without a Receiver, the SlimKIC can still be used as a real-time profiler using the Direct Connect cable. The benefits of this are:
- You can easily see that all of the TCs on a given PCB are working properly by warming the TC up with your finger and watching the appropriate temperature on the PC graph go up.
- With the SlimKIC sitting outside the conveyor oven, you can trail long thermocouples wires through the oven and see a real-time thermal profile of your product. This is especially useful in high temperature applications where the SlimKIC will not survive.
- You can profile rework stations in real-time to quickly determine the proper temperatures and time constraints.
- Because the SlimKIC can display temperature in real-time, it is the only Data Logging Profiler that can be calibrated by the user.
The initial steps for profiling solder reflow or wave solder processes are similar. Oven specific information such as zone lengths and load table length is entered into the PC using the “Process Setup” program (See Figure 3).
The SlimKIC comes with either a TC Yoke or removable Harness. The Yoke has up to 12 inputs for individual TCs, and is compatible with standard miniature Type K thermocouples used by most profilers (See Figure 4).
The TC Harness was developed to speed up the connection process and eliminate connection errors when 6 or more TCs are used. One end of the TC harness plugs into the SlimKIC, and the other end has up to 12 TC’s that attach directly to the PCB (See Figure 4a).
To find the highest and lowest temperatures on the PCB, up to twelve thermocouples are attached to the board. The highest peak is found near the PCBs bare edges and the lowest peak is typically at the larger components, near the center of the PCB (See Figure 5). We suggest using high temperature solder to attach the thermocouples to the PCBs. This requires a sacrificial board, but is the most reliable method of attaching the TC’s. For more information on soldering TC’s to PCB’s, see Application Note #00001. Other options include thermally conductive adhesive or reusable thermocouple probes which easily attach to and detach from the board. For reusable thermocouple probes KIC recommends you contact Saunders Technology Inc.
The ideal solder reflow thermal profile is typically based on three factors: Peak Temperature, Maximum Slope, and Time Above Reflow Temperature.
For a solder with a melting point of 179°-183°C, the minimum allowable peak temperature is usually 195°C-205°C. The maximum allowable temperature peak is 220°C-230°C. If the PCB gets too hot, the edges may turn brown, and temperatures above 230°C can cause damage to components. Conversely, if the PCB does not heat up enough, the solder paste will not adequately reflow.
The maximum slope is usually measured in degrees/second, and specifies how fast the PCB temperature is allowed to change. Some components may crack if their temperature changes too rapidly. The maximum rate of thermal change that the most sensitive components can withstand becomes the maximum allowable slope. In order to maximize throughput, the PCB thermal profile is usually designed to have a maximum slope just under the maximum allowable, typically 2-4°C/sec.
The time above reflow is the measure of how long the solder on the PCB is liquidous. Assemblers generally like to see the solder liquid for between 30 and 60 seconds, although liquidous times of 90 seconds or more are not uncommon on larger boards. If the solder is above reflow temperature too long, excessive growth of tin-copper intermetallics leads to a tin-depleted and brittle solder joint. If the solder is above the reflow temperature for less than 30 seconds, there is a risk that oven temperature fluctuation during production could cause the profile to drop below reflow temperature.
With some no-clean fluxes the “flux activation time” is often a key parameter. For example: the flux becomes active at 130°C, and is used up by 160°C. This makes the rising time between 130°C and 160°C is critical. The KIC Software can calculate this rising time automatically (See Figure 6).
Figure 6: The KIC Software automatically calculates key profile “statistics” such as
peak temperature, and the total time above the solder melting temperature. Colored
boxes allow you to quickly see which statistics are outside your specification
In both solder reflow and wave solder profiling the oven/machine zone setpoints and conveyor speed are set to some first-guess temperature. After the oven has stabilized, a test PCB with TC’s attached is connected to the SlimKIC and sent through the oven. As the leading edge of the PCB enters the oven, the user presses the start button. This tells the SlimKIC software to begin recording PCB temperatures. When the leading edge of the PCB exits the oven, the stop button is pressed, and the KIC Software calculates the actual conveyor speed. If the conveyor speed is off by more than 1%, a warning is appears on the PC screen (See Figure 7). In real-time mode, Peak Temp./Max. Slope, and Time Above Reflow are also displayed on the PC screen as the profile is being taken. The real-time feature allows the user to see board temperatures as they occur. If one of the three factors dramatically changes, the user can change the process immediately. Should the SlimKIC become too hot, an alarm in the computer will sound alerting the user. The SlimKIC continually transmits its own internal temperature as a safety measure.
After the first run, if the PCB thermal profile is not correct, the SlimKIC software has the capability to provide accurate profile prediction. KIC systems use a mathematical model of the oven environment to predict the product profile. The user simply enters changes in setpoints and/or conveyor speed, 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 process by running the test PCB no more than 2-3 times (See Figure 8). Profile Prediction shows that this profile can be brought into “spec.” by lowering zone 9 by 10°C. (See “Auto-Predict” below to eliminate guesswork from oven set-up.)
The key profile parameters for the wave soldering process will depend on the type of flux used. OA (Organic Acid) or RMA (Rosin Mildly Active) fluxes typically have such a wide thermal process window that advanced profiling systems like the SlimKIC are often not used. Instead the user simply adjusts the process based on the “look” of the solder joints or perhaps uses a thermal sticker that changes colors when a key temperature is exceeded. The disadvantage of these fluxes is that the board must be cleaned after it is soldered. “No Clean” fluxes have a much tighter thermal process window and are usually profiled regularly. These profile requirements vary significantly depending on the flux and board type. Usually there is a “flux activation temperature” that the board must rise above for a minimum amount of time. Too low a temperature and/or too short a time at the specified temperature and the flux will not adequately remove the oxides from the board. Too long a time above the specified temperature will cause the flux to be “used up” and cause the board to begin oxidizing. Topside temperatures above 160°C will induce failures in some BGA’s (Ball Grid Arrays).
When the PCB first enters the wave solder machine, it passes through the fluxer and the bottom of the PCB is coated with flux. It then passes over the preheat area, usually made up of 1-3 heated zones. Typically, the PCB should rise no more the 2°C per second in the preheat area, and the temperature of the top side of the board should rise to about 150°C-160°C, or to within at least 100°C of the wave (molten solder) temperature. These are commonly used specifications. We suggest you contact your flux and solder manufacturers for their recommendations.
When passing through the wave solder machine, if the PCB temperature is raised more than 100°C in the wave, the board may warp and component thermal shock may occur. This is especially true of SMT components that are being soldered to the bottom side of the PCB.
The solder in the wave usually has a melting point of 183°C. The wave temperature is usually 249°C-260°C. As the PCB passes over the wave, the bottom of the board is immersed in the solder. The surface tension of the solder causes it to be pulled up into the holes of the PCB, which solders the components into place.
The dwell time over the wave is usually 3-5 seconds. If the dwell time is too short, good wetting will not occur. If the dwell time is too long, the components may overheat. The SlimKIC is capable of taking up to 20 readings per second from each thermocouple on a test board (in data logging mode). Dwell time can be accurately measured by putting a thermocouple through a hole in the board and calculating the time when the TC temperature first starts rising to when it first starts falling. This is a fairly simple task with the Zoom and Pointer tools in the KIC software
When monitoring temperatures with the SlimKIC remote transmitter/data logger, the SlimKIC unit rides over the solder on a board behind the part being profiled, and the SlimKIC itself never touches the solder bath.
After the PCB exits the wave, the temperature drops rapidly and the solder hardens. Some wave solder machines have one or more fans to help cool the PCB before it goes on to the next processing step.
Like solder reflow, when running the first wave solder profile, the machine zone setpoints and conveyor speed are set to some “first guess” temperature, then the board is run through the process. The user then has the initial data for a profile prediction.
A powerful new tool that became available to SlimKIC users in 1997 is the Auto-Predict Software option. Auto-Predict dramatically reduces the time spent searching for the optimal oven recipe (set-points and belt speed) and increases the accuracy of the resulting product thermal profile. It is applicable to all conveyorized processes including solder reflow ovens and wave soldering machines.
Auto-Predict completely eliminates guesswork from oven profiling. The user simply enters the product thermal profile specifications (Peak Temp=215°+-10°C, Max Slope=3.0°C, etc.). The software then calculates the product thermal profiles for thousands of potential oven recipes and ranks them by how well they meet the specifications. In minutes, the user often has 20 or more “in spec.” recipes to choose from and the power to select a profile that is precisely centered in the process window. Auto-Predict can also determine the maximum conveyor speed, and can help find a single recipe to satisfy multiple product types. In Figure 9, Auto-Predict has been used to find a recipe that will allow the belt-speed for the profile in Figure 8 to be increased from 35 to 36 inches/minute for increased throughput.
The SlimKIC gathers thermal information that can be saved, allowing you to generate detailed hard copy reports from most color and black & white printers.
At the wave soldering step, the last few PCB’s in a large run often are not getting good wetting. This problem is typically associated with too low a solder pot temperature, even though the thermostat on the solder pot shows that the solder temperature is correct.
- Causes: Not enough spacing between large PCB’s.
- Solution: Leave a greater distance on the conveyor between large PCBs because the solder pot cannot maintain temperature. The solder pot control thermostat is usually placed deep inside the wave, far from the surface. Tests have shown that after a run of large PCB’s, the temperature at the surface of the wave can drop as much as 12°F (6.6°C) below the control TC temperature.
The wave soldering machine is leaving solder balls on the bottom side.
- Cause #1: Board temperature in the pre-heat area is not hot enough and the water in the water-based flux is not completely evaporated. The water remaining on the board vaporizes in the wave and causes solder balls.
- Solution: Increase the board temperature in the pre-heat zones by increasing the pre-heat temperatures and/or reducing the conveyor speed. Use the Auto-Predict feature of the KIC Software to bring the profile within the flux manufacturer’s specification.
- Cause #2: Board temperature in the pre-heat area of the oven is too hot and all the flux activators are burned off before the board reaches the wave, causing the solder to stick to the bottom of the board as it exits the wave.
- Solution: Reduce the board temperature in the pre-heat zones by decreasing the pre-heat temperatures and/or increasing the conveyor speed. Use the Auto-Predict feature of the KIC Software to bring the profile within the flux manufacturer’s specification.
Annual or semiannual maintenance was just performed on the solder reflow oven and now most of the profile recipes need to be adjusted.
- Cause: With many conveyor ovens and wave solder machines, the conveyor speed fluctuates over time, often deviating as much as 5%. Thus, a conveyor speed of 30 inches/minute could be as high as 31.5 inches/min. or as low as 28.5 inches/min. This may not sound like a big change, but it is enough to push a borderline profile outside of specifications.
Conveyor speed is usually only calibrated once or twice a year. Typically, the user is constantly adjusting the profile over time as the conveyor speed drifts a little each month. For example: after six months, the user is setting the conveyor speed to 31.5 inches/min., when in reality, the conveyor is running at 30 inches/min. At the semiannual maintenance the conveyor speed is calibrated. Now, when the conveyor speed is set to 31.5 inches/min., it is really running at 31.5 inches/min. Every recipe will have changed, and some will no longer reflow the PCBs properly.
- Solution: The belt speed must be checked at least weekly to assure that it is not drifting. The SlimKIC makes this easy because it can check the accuracy of the conveyor speed every time a PCB is profiled. At the first sign of drift, the conveyor speed should be calibrated.
The SlimKIC greatly simplifies the process of obtaining product temperature profiles, providing the user with a more accurate data and a more efficient method of acquiring data on critical thermal processes. With the Auto-Predict software option, the SlimKIC makes it possible to greatly reduce oven set-up time and quickly find an optimal product profile that is precisely centered in the process window.