SMT furnace temperature curve and main failure analysis
In the smt production process, the quality of the reflow oven parameter settings is the key to the welding quality. The temperature curve measured by the furnace temperature tester can provide an accurate theoretical basis for the setting of the reflow oven parameters. In most cases, The temperature distribution is affected by the characteristics of the assembled circuit board, the characteristics of the solder paste, and the capacity of the reflow oven used.
Detailed setting of furnace temperature curve
Using a furnace temperature curve tester is the best way to obtain and establish an applicable temperature curve. When measuring, you must use a fully assembled circuit board. Carefully select a few points on the circuit board. The point may be the most representative (maximum or minimum) in terms of heat capacity, heat conduction and heat absorption. Adhere the temperature sensor head with high temperature tape or solder with high temperature solder on the selected measurement point, and then send the circuit board In the reflow oven, the computer reads the temperature curve through the interface provided by the measuring instrument. The more advanced measuring instrument can be sent into the reflow oven along with the circuit board. The KIC furnace temperature tester can store the temperature curve in the internal memory. After the measurement, it is read out by the computer or printer.
3. The furnace temperature curve is generally divided into four sections:
a. Rapid heating section (ie preheating section),
b. Insulation section,
c. Soldering reflow section,
d. Cooling section, and briefly analyze the forward and reverse directions from the cooling section.
(1) Cooling section
The lead-tin powder in this section of solder paste has melted and fully wets the surface to be soldered. Rapid cooling will result in bright solder joints with a good shape and low contact angle. Slow cooling will make the plate dissolve in the solder. The formation of dull and rough solder joints may cause poor soldering and weaken the bonding force of the solder joints.
(2) Welding section
This section brings the circuit board above the melting point of the lead-tin powder, allowing the lead-tin powder particles to combine into a tin ball and fully wet the surface of the soldered metal. Bonding and wetting are carried out with the help of flux. The higher the temperature, the higher the efficiency of the flux, and the viscosity and surface tension decrease with the increase of temperature, which promotes the solder to wet faster. However, too high temperature may cause heat damage to the board, and may cause problems such as accelerated reoxidation of lead-tin powder, scorching of solder paste residue, discoloration of the board, and loss of function of components. Too low temperature will make the flux inefficient. It is possible that the lead-tin powder is in a non-soldering state and increases the probability of raw welding and virtual welding. Therefore, the optimal combination of ideal peak value and time should be found. Generally, the coverage area of the tip area of the curve should be minimized. The duration of reaching the peak temperature is 3-5 seconds, and the duration of exceeding the melting point temperature of the alloy is maintained between 20-30 seconds.
(3) Insulation section
The boiling point of the solvent is between 125-150℃. The solvent will continue to evaporate from the heat preservation section. The resin or rosin will begin to soften and flow at 70-100℃. Once melted, the resin or rosin can quickly diffuse on the surface to be welded and dissolve in it. The active agent flows and reacts with the surface oxides of the lead-tin powder to ensure that the lead-tin powder is clean when the soldering section is welded. The main purpose of the heat preservation section is to ensure that all components on the circuit board reach the same temperature before entering the soldering section. The heat absorption capacity of the components on the circuit board is usually very different. Sometimes it is necessary to extend the heat preservation period, but the heat preservation period is too long It may cause the loss of flux, so that the welding area cannot be fully bonded and wetted, and the soldering ability of the solder paste is weakened. Too fast temperature rise rate will cause rapid vaporization of the solvent, which may cause blow holes, tin beads, etc. Defects, and a too short heat preservation period will not make the active agent fully effective, and may also cause the entire circuit board to be lower than the preheating temperature. It is recommended to be between 100-160℃, and the rate of rise is less than 2 degrees per second. There is a platform for about 0.5-1 minutes at about 150°C to help minimize the tip area of the welding section.
(4) Accelerated heating section
The purpose of this section is to heat the circuit board at room temperature as soon as possible, but the rapid heating cannot be as fast as the damage of the board or parts and the loss of solvent in the flux. The usual heating rate is 1-3°C/sec. In actual production, the curve at each point selected cannot be required to achieve a more ideal situation. Sometimes due to the difference in component density, the highest temperature and the huge difference in thermal characteristics, or due to the difference of the plate and the capacity of the reflow oven Restrictions will cause the temperature curve at some points to fail to meet the requirements. At this time, the most favorable reflow parameters must be selected by integrating the effects of various components on the function of the entire circuit board.
Contact Person: Mr. Ivan Zhu