How to set solder paste reflux temperature curve

Correct temperature curves will ensure high quality solder spots.

I. Test method

An optimized reflux temperature profile is one of the most important factors in achieving good solder joints in printed circuit board (PCB) assemblies using surface mount components. The temperature curve is a function of the temperature applied to the circuit assembly as a function of time. When plotted in the Cartesian plane, a curve is formed representing the temperature at a particular point on the PCB at any given time during reflux.

Several parameters influence the shape of the curve, the most critical of which are the conveyor belt speed and the temperature Settings for each zone. The belt speed determines the duration for which the board is exposed to the temperature set for each zone. Increasing the duration allows more time for the circuit assembly to approach the temperature set for that zone. The sum of the duration of each zone determines the total processing time.

The temperature setting of each zone affects the temperature rise rate of the PCB, and high temperature produces a large temperature difference between the PCB and the temperature of the zone. The set temperature of the added zone allows the board to reach a given temperature faster. Therefore, a graph must be made to determine the temperature curve of the PCB. Next is an outline of the steps used to generate and optimize the graph

Before starting the curve step, you need the following equipment and auxiliary tools: temperature curve meter, thermocouple, tool to attach thermocouple to PCB and solder paste parameter sheet. Available from most major electronic tool suppliers, the temperature curve accessory kit makes curve making easy because it contains all the necessary accessories (except the curve meter itself).

Many reflow soldering machines now include an on-board thermometer and even smaller, inexpensive countertop stoves. Thermometers are generally divided into two categories: real-time thermometers, real-time transmission of temperature and time data and graphics; The other takes samples and stores the data, which is then uploaded to a computer

The thermocouple must be of sufficient length and withstand typical furnace temperatures. Generally smaller diameter thermocouple, small thermal mass, quick response, accurate results.

There are several ways to attach a thermocouple to a PCB, preferably using a high-temperature solder such as a silver-tin alloy with minimal solder joints.

Another acceptable method, quick, easy and accurate enough for most applications, covers the thermocouple with a small amount of a thermal compound (also called a thermal conductive paste or thermal grease) in spots and holds it together with a high-temperature tape (such as Kapton).

Another way to attach a thermocouple is to use a high temperature adhesive, such as a cyanoacrylate binder, which is usually less reliable than other methods. The location of attachment should also be chosen. It is usually best to attach the thermocouple tip between the pad and the corresponding component pin or metal end.

FIG. 1. Attach the thermocouple tip between the pad and the corresponding component pin or metal end.

A solder paste characteristic parameter list is also necessary, containing information that is critical to the temperature curve, such as the desired duration of the temperature curve, the active temperature of the solder paste, the melting point of the alloy, and the desired maximum temperature of reflux.

Before we begin, we must have a basic understanding of the ideal temperature curve. The theoretically ideal curve consists of four sections or sections, with the first three heated and the last cooled. The more the furnace temperature zone, the more accurate and close to the temperature curve profile. Most solder pastes are reflow successfully with four basic temperature zones.
FIG. 2. Theoretically ideal reflux curve consists of four zones, with the first three zones heated and the last zone cooled.

The preheating zone, also known as the ramp zone, is used to raise the temperature of the PCB from the ambient temperature to the desired active temperature. In this region, the product temperature rises continuously at a rate of no more than 2 to 5°C per second. Too high a temperature will cause some defects, such as fine cracks in ceramic capacitors, while too low a temperature will cause the solder paste to overheat and not have enough time to reach the active temperature of the PCB. The preheating zone of the furnace generally accounts for 25~33°% of the length of the whole heating channel.

The active zone, sometimes called dry or wet zone, generally accounts for 33~50°% of the heating channel. It has two functions. The first is to sense the temperature of the PCB at a fairly stable temperature, allowing components of different masses to be homogeneity in temperature and reducing their considerable temperature differences. The second function is to allow the flux to be activated and volatile substances to evaporate from the solder paste. The general active temperature range is 125 to 150° C. If the active zone temperature is set too high, the flux does not have enough time to activate, the slope of the temperature curve is an upward slope. Although some solder paste manufacturers allow some temperature increase during activation, the ideal curve requires a fairly smooth temperature so that the temperature of the PCB is equal at the beginning and end of the active zone. Some ovens on the market cannot maintain a flat active temperature curve. Selecting one that can maintain a flat active temperature curve will improve weldability and allow the user a larger processing window. The reflux zone is sometimes called the peak or final heating zone. The function of this zone is to raise the assembly temperature from the active temperature to the recommended peak temperature. The activity temperature is always a little lower than the melting point of the alloy, and the peak temperature is always at the melting point. The typical peak temperature range is 205 °C to 230°C. Setting the temperature too high in this region can cause the temperature rise slope to exceed 2 °C to 5°C per second, or reach a peak reflux temperature higher than recommended. This can cause excessive crimp, delamination or burn of the PCB and damage the integrity of the component.

Today, the most commonly used alloy is Sn63/Pb37. This ratio of tin and lead makes the alloy eutectic. Eutectic alloys are alloys that melt at a specific temperature. Non-eutectic alloys have a melting range, not a melting point, sometimes called a plastic state. All examples described in this paper refer to eutectic tin-lead, which has a melting point of 183°C because of its widespread use

Ideally, the cooling zone curve should mirror the reflux zone curve. The closer the mirror relationship is, the closer the structure of the solder joint reaches the solid state, the higher the quality of the solder joint and the better the integrity of the bonding.

The first parameter to consider in making a temperature curve is the speed setting of the transfer belt, which determines the amount of time the PCB spends in the heating channel. Typical parameters of solder paste manufacturers require heating curve of 3 ~ 4 minutes, with the total channel length divided by the total thermal heating time, heating is the accurate transmission belt speed, for example, when the solder paste for 4 minutes of heating time, using six feet heating channel length, the calculation for the present six feet four minutes = 1.5 feet per minute = 18 inches per minute.

Next you must decide on the temperature Settings for each zone. It is important to understand that the actual zone temperature is not necessarily the displayed temperature for that zone. The displayed temperature only represents the temperature of the thermocouple in the region. If the thermocouple is closer to the heating source, the displayed temperature will be higher compared to the temperature of the region. The closer the thermocouple is to the direct channel of PCB, the displayed temperature will be more able to reflect the temperature of the region. It is advisable to consult the furnace manufacturer to understand the relationship between the displayed temperature and the actual temperature range. Interval temperatures rather than display temperatures are considered in this article. Table 1 lists the interval temperature Settings for reflux in a typical PCB assembly.

Table 1. Interval temperature setting of typical PCB reflow

Once the speed and temperature are determined, they must be input to the furnace controller. Check the manual for other parameters that need to be adjusted, including cooling fan speed, forced air shock, and inert gas flow. Once all parameters are entered, the machine is started and the furnace is stable (i.e., all actual displayed temperatures are close to conforming to the set parameters) the curve can be started. The next one places the PCB on a conveyor belt, which triggers the thermometer to start recording data. For convenience, some thermometers include a trigger function that automatically starts the thermometer at a relatively low temperature, typically slightly higher than the human body temperature of 37°C(98.6°F). The 38°C(100°F) automatic trigger, for example, allows the thermometer to operate almost as soon as the PCB is placed on the conveyor belt into the furnace, preventing the thermocouple from accidentally triggering when handled by human hands.

Analysis of test results

Once the initial temperature profile has been produced, it can be compared with the paste manufacturer's recommended curve or the curve shown in Figure 2.

First, it must be verified that the total time from ambient temperature to peak reflux temperature is compatible with the desired heating curve residence time, increasing the belt speed proportionally if it is too long, and vice versa if it is too short.

Next, the shape of the graph curve must be compared to the desired shape (Figure 2), or if the shape does not match, to the figure below (Figure 3-6). Select the curve that best matches the shape of the actual graph. Deviations from left to right should be considered. For example, if there is a difference between the warm-up and backflow zones, adjust the warm-up zone difference correctly first. It is generally best to adjust one parameter at a time and run the curve setting before making further adjustments. This is because changes in a given region will also affect the results of subsequent regions. We also recommend fairly small adjustments for beginners. Once you have gained experience with a particular furnace, you will have a better "feel" for how much adjustment you can make.

FIG. 3 underheated or overheated reflux curves

Figure 4. Too high or too low temperature in the active area

FIG. 5. Too much or not enough reflux

FIG. 6. Cooling too fast or not enough

The furnace parameters should be recorded or stored for later use when the final curve is as close as possible to the desired pattern. Although this process is slow and laborious at first, eventually proficiency and speed can be achieved, resulting in efficient production of high-quality PCBS.