Reflow Thermal Profiling Print E-mail
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Written by ACI Technologies Inc.   
Wednesday, 19 May 2010 08:18

Implementing a successful reflow process from scratch.

Reflow temperature profiling is the most important aspect of proper control of the soldering process. It may appear to some an art, practiced by a select experienced few who are able to divine the proper settings for a reflow oven by reading graphs as if they were tea leaves. This does not have to be true. Here, we outline a systematic method to implement a successful reflow process from scratch.

The most basic type of profile is a ramp-to-peak (RTP) profile (Figure 1). This profile type is one where the rate of temperature increase over time is virtually constant for the entire heated portion of the profile. An RTP profile type is common and is the easiest to implement. There are three critical parameters for all solder materials on an RTP profile: peak temperature, rate of temperature increase over time (slope), and time above liquidus (TAL).

Peak temperature is exactly what it appears: the highest temperature experienced during reflow. Slope is the rate of temperature increase over time during the reflow process. TAL is the time spent above the temperature at which the solder alloy is fully melted. These parameters vary based on the alloy (especially peak temperature and TAL) and flux formulation (especially slope). The primary source for these parameters is the solder paste manufacturer’s data sheet. In many cases, these specifications will provide an acceptable range. In some cases, only a minimum or maximum requirement is provided. For our purposes, we will use a fictional solder paste that provides the following requirements: peak temperature of 240°-255°C, profile slope of 0.8°-1.0°C/sec., and a TAL of 30-60 sec.

The first step when developing a reflow profile is to set the conveyor speed. This is the most important parameter to set correctly, as any change during process development will invalidate all the work accomplished to that point. The conveyor speed can be calculated, provided all the necessary information is available. The technician must know (or measure) the heated length of the oven and determine the required peak temperature and profile slope.

The next step is to calculate the time needed to reach the peak temperature by determining the difference between the peak temperature and room temperature and dividing that result by the slope. In our hypothetical example, the time to peak is (247.5 - 25) / 0.9 = 247.2 sec. Notice the midpoint was used for each range. This ensures the calculated conveyor speed is near the center of the acceptable range.

Once the time to peak has been determined, the conveyor speed is calculated by dividing the heated length of the oven by the time to peak. Our hypothetical oven has 84˝ of heated length, resulting in a conveyor speed of 84 / 247.2 = 0.34˝/sec. or approximately 20˝/min. The precision of the conveyor speed setting is not critical because the center of the range was used for peak temperature and profile slope, so rounding the value is acceptable. Once this value is determined, it will remain unchanged for the balance of the profile development.

The next task is to determine the goal temperature for the assembly at the end of each oven zone. To calculate the goal temperature at the zone exit, the following must be known: the number of heated zones in the oven, the peak temperature desired, and the exit temperature of the previous zone. The calculation begins by determining the desired temperature rise for each zone, calculated by dividing the difference between peak temperature and room temperature by the number of heated zones. In our example, the oven has seven heated zones, so the calculation is (247.5 - 25) / 7 = 31.8, or approximately 32°C per zone.

The goal temperature for zone 1 is then calculated by adding the previous zone exit temperature (room temperature for zone 1) and the temperature rise per zone. For our example, this becomes 25 + 32 = 57°C. This is the temperature the assembly should reach by the end of the first zone, but the oven should be set to a higher value. There will be a difference between the oven set point and the temperature of the assembly during the reflow process. A good starting point is approximately 20°C higher, so the oven’s first zone should be set to 80°C. The subsequent zones can remain at their default value (typically room temperature) for now. Once the first zone has reached operating temperature, a measurement can be taken by passing an assembly with thermocouples and a data logger through the oven. After each pass, the assembly’s temperature is compared to the goal, and the oven set point is adjusted as necessary, until the assembly exits the first zone at approximately 57°C.

This process is repeated for each zone in sequence.

Ensure the slope of the profile curve remains constant throughout the zone. A profile that flattens at the end of any zone indicates the assembly is nearly reaching temperature equilibrium in that zone. This can be due to a high convection rate, which should be reduced, if possible. If the oven does not have adjustable convection rates, increase the conveyor speed. If the conveyor speed is changed, recalculate the expected slope to ensure it is within specification. This is accomplished in the same manner as the determination of the conveyor speed, except the conveyor speed is now a known value, and the expected slope is the unknown value. If the conveyor speed is changed, the entire zone setting process should restart from zone 1.

Typically, the final two (or three) zones are where reflow occurs, and is where the profile should exceed the liquidus point of the solder. The entire time the profile spends over the liquidus point of the solder is counted toward the TAL parameter. This includes the time after the peak temperature (which will occur at the end of the last heated zone). The peak temperature and TAL typically are adjusted by modifying the temperatures of the last two or three zones. This is accomplished through trial and error. However, by following this system, trial and error is limited to minor changes in a limited number of zones at the end of the process.  CA

ACI Technologies Inc. (aciusa.org) is the National Center of Excellence in Electronics Manufacturing, specializing in manufacturing services, IPC standards and manufacturing training, failure analysis and other analytical services. This column appears monthly.

Last Updated on Wednesday, 19 May 2010 08:25
 

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