Thermal Profiling the Reflow Process

Transcription

1 Thermal Profiling the Reflow Process The Nomadics TCProfile system is a cost-effective instrument to measure the temperature characteristics of any process where the thermal profile is important to the quality and performance of the product. Designed for specific use with conveyor ovens, common applications include: A. SMT Reflow Soldering B. Through-Hole Wave Soldering C. Epoxy Curing D. Thick Film Firing E. Electronic Circuit Rework F. Metal Brazing G. Food Processing Thermal profiling is used to set up and monitor processes to ensure product conformance. The manufacture of surface mount electronic assemblies is a typical application where profiling the reflow process is key to improving first pass yield and minimizing rework. With the increasing use of no clean fluxes and lead free solders thermal profiling becomes more important. Profiling assemblies provides for higher yields and less rework as well as records the process for customer verification and statistical process control. Background Today s modern reflow ovens are provided with thermal instruments to monitor any change in the oven s performance resulting from a change in the heater efficiency, airflow and conveyor speed, but users need to know the heat transfer from the oven to the assembly. Mounting thermocouples on a populated assembly in process will provide the critical profile. So what makes a good profile? Unfortunately there is no best reflow profile that covers all PCB assemblies. A typical solder paste profile is shown in Figure 1.

2 Typical Profile for Solder Alloy Sn 63 Pb 37 Maximum Rising Slope Minimum Falling Slope 3 degc/sec -6 degc/sec Temperature Degrees C Zone1 PreHeat Zone2 Soak Zone3 Reflow Zone4 Cool Down sec sec sec Time Zone 1 PCB and components are heated to maintain a temperature gradient less than 2 4 ºC/sec. Zone 2 Flux begins activation and different mass components reach temperature equilibrium. Zone 3 Solder paste reaches melting point. Zone 4 Cooling process begins. Figure 1 Typical Solder Paste Profile Setting the conveyor speed and zone temperatures to the profile recommended by the solder paste manufacture is a starting point but knowing the heat transfer from the oven to the populated assembly is the key to success. This heat transfer is determined by 1) the PCB size, thickness, number of layers, layout and 2) the components density, mass, size, package, material, thermal resistance. The process window obviously varies with each assembly. A properly set profile can minimize rework and improve long term reliability. Table I provides a summary of the problems found with profiles not matched to the assembly. Problem Brittle Solder Joint Cold/Grainy Solder Joint Components Cracking Component Degradation or Failure Discolored Joint Inadequate Solder Flow Leaching PCB Discoloration PCB Delaminating PCB Warpage Solder Balls or Splatter Tombstoning White Residue Table I Reflow Problems Probable Cause Excessive Time Above Reflow Low Peak Temperature Excessive Temperature Change Excessive Peak Temperature Excessive Soak Time Low Peak Temperature Excessive Reflow Time or Temperature Excessive Peak Temperature Excessive Peak Temperature Excessive Peak Temperature or Cool Down Rate Low Preheat Temperature, Excessive Ramp Up Excessive Temperature Change Excessive Time Above Reflow Temperature

3 The hottest areas of the assembly are normally the edges of the board and the low mass components while the coolest areas are normally the center of the board and any high mass components. An incorrect thermal profile can result in the rework or scrapping of assemblies driving up the manufacturing cost. Using the capabilities of TCProfile aids in minimizing problems with the reflow process. TCProfile Setup When using TCProfile for the first time the oven and test parameters must be specified. The operating screen for a new profile that has been setup is shown in Figure 2. Multi-Zoned Ovens Selecting New Profile from the File menu provides the initial setup for a new profile. A series of three windows will step you through setting up a new assembly to be profiled. Step one also provides for adding or editing an oven. All test settings can be specified as English/Metric/Fahrenheit/Celsius and saved. Figure 2 New Profile Setup Screen Single-zone Oven with No Conveyor or Batch Oven Use the New Profile from the File menu as with a multi-zone oven and then follow the following example. 1. Define a new oven. Enable Z0 = 0 inches and Z1 = 120 inches. Zone Z0 is assumed to be the entrance to the oven and out of the heating zone so always use Z1 for the heated zone. 2. A conveyor speed must be set even though there is no conveyor. Set the conveyor speed to match the length of the test if you want the profile to automatically stop i.e. for a test time of 15 minutes set the conveyor speed to 8 inches/minute and the test will stop

4 in 15 minutes. If you want to manually stop the oven set the conveyor speed to some arbitrarily long time such as 1 inch/hour and it will not stop for 120 hours. 3. Set all other parameters as for a multi-zoned oven. After completion a profile can be run with the defined settings and once this profile is saved it may be quickly recalled and a new profile run with the same settings. Saving the settings allows the user to duplicate the same conditions at a later time or to modify each test characteristic independently to tweak the profile characteristics. More detail on setting up TCProfile for optimum performance is described in the Users Manual and Frequently Asked Questions documents. Running a Profile Up to six thermocouples may be placed on the assembly and proper selection of their locations is important. Unless there are specific known problem areas the thermocouples should be placed in areas to provide a) maximum peak temperature, b) maximum rate of temperature change and c) maximum and minimum dwell above wetting temperature. The maximum peak temperature will normally occur along the edges of the PCB near low mass components such as chip resistors or capacitors. The minimum peak temperatures will normally occur near the center of the PCB near high mass components such as large IC packages. Thermocouples may be attached using Kapton tape, high temperature solder or thermally conductive epoxy. If the thermocouples come loose from the PCB then the reading will be of the air temperature and provide no value in the thermal characteristics of the PCB assembly. Kapton tape is convenient to use and does not damage the PCB but can come loose and provide inaccurate data. When using Kapton tape a sudden discontinuity in the profile is an indication of poor thermocouple adhesion. Use of high temperature solder and conductive epoxy is repeatable and the thermocouples may be placed directly on solder pads but the PCB will probably be destroyed. TCProfile provides a real time profile of the assembly but care must be exercised that the trailing thermocouple bundle does not become entangled in the conveyor mechanism. In addition the PCB typically rides on a rail above the conveyor and careful feeding of the thermocouples through the oven is required to assure that the assembly is not slowed by tension on the thermocouple leads. TCProfile is provided with a bundle of six thermocouples in a fiberglass sheath to minimize both problems. After the PCB is placed on the rail carefully feed the thermocouples minimizing drag on the PCB. After the PCB has traversed 12 to 18 inches into the oven the thermocouples can be placed on the conveyor and the thermocouple bundle fed using less care. TCProfile s thermocouple bundle is comprised of a 16 foot extension section with male connectors on one end for connection to the PC Card and female connectors on the opposite end for connecting to the measurement section that is 2 feet long and has exposed thermocouples for connection to the PCB. At the end of the profile the measurement section may be disconnected and left attached to the PCB and the extension section pulled back to the oven entrance. Viewing the Real Time Profile Placing the assembly on the conveyor and selecting Start starts the profile. The data is available for review (Figure 3) as the assembly travels through the oven.

5 Figure 3 In-Process Profile Temperature: The temperature shown is for the last data point measured. Slope: The slope is for the last data point measured. The slope is calculated from the last two data points. Time Above Temperature (TAT): The amount of time a channel has spent above the operator set reference. Ref Temp: Operator set reference temperature for calculating time above temperature Time Above Temp: The combined time any combination of channels spent above the reference temperature. Max Slope: The maximum positive or negative slope any channel encountered during the measurement. Graph: Using the mouse pointer, panning and zooming can be done at any time measurements are in process or after measurements are complete. Panning With the mouse pointer on the graph and holding the right mouse button down the graph may be panned. Zooming With the mouse pointer on the x or y-axis and holding the left mouse button down while moving the mouse pointer along the axis allows the axis to be zoomed.

6 Restore - After panning or zooming the axis can be reset to optimize the view by placing he mouse pointer on each axis, holding the control button down and left clicking the mouse button. Status Bar, S/N: Shows the serial number of the card installed while measurements are in process. Status Bar, Time: The time section is not active while measurements are in process. After the test is stopped, left clicking the mouse on the graph will draw a line at that point and show the position at that location. Status Bar, Description: While measurements are in process this will display Collecting Data. After measurements are stopped it will display Viewing Data. When viewing saved measurements it will display Viewing Data (File name and Location). At any time the mouse pointer is placed over the colored Channel number text in the Channel Readings section it will display the channel text provided by the operator under Profile settings. Analyzing the Profile After completion of the test the graphic of the profile can be saved for easy recall and comparison with other profiles. Typical of the analysis that can be performed are: Specifying a new temperature reference for time above temperature reference. Using Profile Settings a new temperature reference may be specified to calculate time above temperature for that reference. Left clicking the mouse on the graph will draw a vertical line at that point and display he temperature and slope at that location. The slope displayed is the slope intersecting the vertical line and the next data point. Using the mouse pointer, panning and zooming can be done as previously described. The graph settings may also be changed to provide zooming. Overlaying a second profile to compare temperature and temperature difference at any location. Figure 4 shows two profiles overlaid. Locate the cursor at any position on the profile, left click the mouse and a vertical line will be drawn at that location. In Figure 4 a point was specified at inches into the profile. In the Channel Readings section the temperature for each channel and the temperature difference for corresponding channels is specified. Under the File menu you can load a saved profile using Load Profile and a second profile using Load 2 nd Profile. All the readings shown in Monitored Values are for the first data set.

7 Figure 4 Two Overlaid Profiles Caution: When comparing two profiles the x and y-axis must have the same units. Changing the Profile After a profile has been run and analyzed it may be desirable to eliminate potential problem areas or to change the profile to better fit a desired curve. The number and length of zones in an oven are fixed and can not be changed but the conveyor speed and zone temperatures can be changed to tweak performance. Decrease Conveyor Speed Increase Conveyor Speed Reduce Zone Temperature Increase Zone Temperature The assembly would spend more time in each zone. This will reduce the rate of temperature change and increase the probability that the assembly will reach the set zone temperature. The assembly would spend less time in each zone. This will reduce the probability the assembly will reach the zone set temperature and increase the probability of a rapid rate of temperature change as the assembly enters the next zone. This will reduce the maximum temperature reached in a zone. It will also effect the rate of temperature change from the adjacent zones. This will increase the maximum temperature reached in a zone. It will also effect the rate of temperature change from the adjacent zones.