March 22, 2021
Seprays Fully automatic PCB splitter share Effect of Temperature Curve on Cleanness and Electrical Performance-Pursuing Reliability
Temperature curve of reflow welding process is one of the most important considerations when setting assembly parameters. In order to obtain an effective temperature curve, factors need to be considered include selecting the appropriate equipment, fully understanding the results and being able to adjust on demand. For some large multi-layer components and components with larger thermal quality, the minimum recommended temperature for forming correct solder joints should be ensured in all areas of the components, and the cleaning-free flux residues should be benign. It is necessary to carefully check assembly drawings of components to determine whether there is a thick copper layer in the selected area. Thicker copper layers absorb heat from the surface of the assembly. This may result in cold and brittle defects in solder joints.
Characteristics of Temperature Curve
Four different stages or regions need to be analyzed under the reflux temperature curve (Fig. 1). First, the preheating temperature rise slope (temperature rise slope), then the preheating temperature residence stage (soaking time), then the time above the liquidus temperature including the peak temperature, and finally the cooling zone.
Fig. 1. Examples of four stages of lead-free reflux temperature curve.
For the flux used in this experiment, it is necessary to control the slope of the preheating slope within 2.0 C/s, which can make the flux evaporate gradually and obtain better quality solder joints. This will not increase the risk associated with solder defects, such as solder ball, short circuit and other defects.
During the residence stage of preheating temperature, the flux activator removes the oxide and connects the metal surface with solder paste. This stage allows the entire assembly of many components to enter a common temperature below the solder melting point. For most solder paste types, this temperature usually needs to be maintained for 60 to 90 seconds.
The reflux stage is the formation of intermetallic compounds. The reflux temperature is usually 20 - 40 C higher than that at the solder melting point. The time to remain above the liquidus is 30-90 seconds, depending on the thermal quality and the choice of other materials (Fig. 1).
Cooling zone is helpful to determine the integrity of solder joint grain structure. Compared with the temperature rise slope in the preheating stage, the cooling slope with a faster temperature drop is usually required, but it is important not to exceed the thermal expansion coefficient (CTE) on the surface of components and circuit boards. The conventional suggestion for cooling temperature drop rate is no more than 4 C/s.
Temperature Analysis Equipment
According to different requirements, there are several kinds of reflux temperature analysis equipment available. Some temperature analyzers are used to analyze products, others are used to analyze reflow furnace. In this study, I only focus on the product temperature analysis equipment that can move with the product. It does not need a long cable, as long as it meets the length of reflow furnace. The product temperature analyzer can measure the temperature of multiple locations on the assembly. Most commercial mobile temperature analyzers use up to six independent thermocouples. Some thermocouples measure data in real time and send it to receivers on computer monitors. Others use internal memory to store data points. When the product leaves the reflow furnace, it is downloaded to the computer storage medium. Both types of data can be used to obtain the required analysis results.
In this experiment, several temperature curves are used to determine the influence of temperature on Cleanliness Measurement and resistance measurement. We use ion chromatography (IC) to measure cleanliness. Surface insulation resistance (SIR) is measured with five volts bias at 40 degree C and 90% relative humidity, once every 10 minutes. All test circuit boards are used to determine whether two kinds of qualified test circuit boards are qualified (Figure 2).
Fig. 2. Two test identification boards for identifying two test results.
The temperature of the first group of temperature curves is 20 degree C lower than the recommended temperature limit. The second set of temperature curves is 10 C lower than the recommended temperature limit. The third set of temperature curves is recommended by the manufacturer for this solder paste. The temperature of this temperature curve is considered to be the lowest allowable temperature at which the correct solder joints and activators are all activated. The fourth group of temperature curves is 10 degrees C higher than the recommended temperature limit.
Under each group of temperature curves, 10 circuit boards are disposed, of which five are tested by IC and the other five are tested by SIR. Measure four different locations of each circuit board, including LCC, TQFP, BGA, and uninstalled line connectors for reference.
As mentioned earlier, these experiments used two analytical techniques: ion chromatography (IC) test and surface insulation resistance (SIR) test.
All IC tests were performed using the Dionex ICS 3000 chromatographic system of Chromeleon software. Automated local extraction technology is used to extract samples. All parts are removed mechanically and samples are extracted from the circuit board layer. Local extraction is very important because it does not normalize the contamination in the whole surface area of components as other extraction methods and test methods do. IC was used to test the original sample of the selected solder paste and determine the main components of the activator in the solder paste. When determining the ion cleanliness level of the PCB plane, the measured data of all samples should be compared with the IC data of the original solder paste.
SIR test is carried out in standard environment warehouse, which can control the change of temperature between (+ 1 degree C) and the change of relative humidity within 3%. Electrical measurements are made with a calibrated automatic switch measuring system, which is measured every 10 minutes.
The first set of temperature curves
The first set of temperature curves is reflux in the worst case when the temperature is 20 degrees C below the recommended temperature by the solder paste manufacturer. When the solder paste is refluxed with this set of temperature curves, it barely reaches the liquid phase state. This may be the result of multiple failed heating sources, incorrect methods (possibly standard lead-containing temperature curves) or other unknown causes (Figure 3).
Fig. 3. The first set of temperature curves.
IC Results of the First Group of Temperature Curves
The original solder paste IC samples show that acetate, chloride, lithium, sodium, ammonium and potassium are the most concentrated ions, and also the most concerned ions after each change of reflux temperature curve. This worst temperature curve shows that the levels of acetate, chloride, lithium and sodium ions have increased significantly after reflux. Using this curve, the contents of ammonium and potassium decreased significantly (Table 1).
Table 1. IC data of the first set of temperature curves.
SIR Results of the First Group of Temperature Curves
The SIR results of the first set of temperature curves show that the surface resistance of all samples does not meet the IPC 1.0e8 ohm limit (Fig. 4).
Fig. 4. SIR results of the first set of temperature curves.
The second group of temperature curves
The second group of temperature curves is closer to the actual situation than the first group. It shows that the influence of flux residue on IC and SIR analysis after reflux when the temperature of temperature curve is only 10 C lower than the recommended temperature limit. However, for circuit boards and components with large thermal mass, it is not impossible to have a temperature change of 10C degree. The test results of these temperature curves emphasize the importance of temperature distribution of components and devices during reflux. Therefore, the equipment must also be tested to ensure that all heaters are working properly. The temperature curves are shown in Fig. 5, IC data in Table 2 and SIR data in Figure 6.
Fig. 5, the second group of temperature curves.
Table 2. IC data of the second group of temperature curves.
Fig. 6. SIR results of the second group of temperature curves.
IC Results of the Second Group of Temperature Curves
The IC results of the second set of temperature curves show that although most ions are relatively low, most of them are still higher than the recommended limits. Especially acetate, lithium ion and sodium ion, the level of these ions in the normal field service environment may increase the risk of failure.
SIR Results of the Second Group of Temperature Curves
The positions of all components do not meet SIR standards, nor do they meet recommended IC limits. In view of the very small heat quality, the connector area (without installation components) has passed SIR and IC tests.
The third group of temperature curves
The temperature curve recommended by the manufacturer with the maximum slope and a rise rate of less than 2 C per second resides at the peak temperature for 30 to 90 seconds. In this paper, the peak temperature is 250 C, and the residence time is about 60 seconds. This temperature curve is shown in Figure 7, IC results are shown in Table 3, and SIR results are shown in Figure 8.
Fig. 7 and group 3 temperature curves.
Table 3. IC data of the third group of temperature curves.
Fig. 8 and the result of SIR of the third group of temperature curves.
IC Results of the Third Group of Temperature Curves
Using the temperature curve recommended by the manufacturer, all flux activators begin to work in the test area, whether or not components are present. Observing the third set of temperature curves, all regions reach at least 246 C, which falls just above the recommended temperature range of 25-45 C above the solder melting point.
SIR Results of the Third Group of Temperature Curves
With or without components, all locations pass acceptable standards. The data in Figure 8 show that no resistance value below 1.0e8 ohm has been observed in the resistance measurement. This means that in the normal practical environment where the atmosphere is not excessively humid, adding voltage will not lead to product failure.
Fourth group of temperature curves
The final set of temperature curves is refluxed at a peak temperature of 260 C to determine (if any) the effect of additional heat energy on cleanliness and SIR performance. The pre-heating rise slope and cooling drop slope of this set of temperature curves are still within the recommended range (Fig. 9).
Fig. 9 and group 4 temperature curves.
IC Results of the Fourth Group of Temperature Curves
Additional heat energy makes the peak temperature exceed 250 C, and does not significantly reduce the ion level. Increasing thermal energy may actually be harmful, causing damage to certain types of components. The data in Table 4 show that the IC results of this group of tests are similar to those of the third group.
Table 4. IC data of the fourth group of temperature curves.
SIR RESULTS OF THE FOURTH GROUP OF TEMPERATURE CURVES
Like the third set of temperature curves, all positions with or without components passed acceptable standards. Figure 10 shows that no resistance less than 1.0e8 ohms has been measured in resistance measurements. This means that in the normal practical environment where the atmosphere is not excessively humid, adding voltage will not lead to product failure.
Fig. 10, the results of SIR of the fourth group of temperature curves.
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