PCB Technical - PCB assembly welding cooling process analysis

PCB assembly welding cooling process analysis
The analysis of PCB assembly welding cooling process mainly talks about the process analysis of PCB assembly welding cooling. Below, please see the specific content introduced by Baiqiancheng electronic engineers.

(1) PCB assembly welding from peak temperature to freezing point.

This area is the liquid phase zone, and a too slow cooling rate is equivalent to prolonging the time above the liquidus line, which will not only make the IMC thicken rapidly, but also is not conducive to the formation of the microstructure of the solder joint, and has a great impact on the quality of the solder joint. For example, when soldering lead-free Sn-Ag-Cu materials with PCB pads impregnated with Sn or Cu/OSP coating, a slower cooling rate will increase the production of Ag3sn and Cu6Sn5; Sn-Ag-Cu solder and ENIG pads Can promote the formation of NiSn4. A faster cooling rate is beneficial to reduce the formation rate of IMC.

Rapid cooling near the freezing point (between 220°C and 200°C) is beneficial to reduce the plasticity time range during the solidification of non-eutectic lead-free solder. For example, the melting point of Sn-Ag-Cu solder is between 20°C and 216°C, and the plasticity time range is short. Rapid cooling and solidification is conducive to the formation of fine crystal particles and the densest structure, which is conducive to improving the strength of SMT solder joints. Shortening the time that the PCB assembly board is exposed to high temperatures is also beneficial to reduce the damage to the thermal components.

Some studies have done a series of process experiments on various cooling slopes, one of which is like this; a specific assembly board is divided into two groups, and two different cooling speeds are used for reflow soldering. The heating rate and preheating time of the first two temperature zones of these two groups are exactly the same, except that two completely different cooling rates are used in the liquid zone. The first group adopts slow cooling, the second group adopts rapid cooling, and then Compared.

It can be seen from the table that in the liquid phase zone, rapid cooling can shorten the liquid phase time, reduce the temperature difference (T) between the largest and smallest components on the PCB surface, and inhibit the growth rate of IMC.

Explained the theory that the rapid cooling of the liquid zone can reduce the largest and smallest components on the PCB surface: In the case of rapid cooling, the heat energy is dispersed into the furnace, but rarely stays in the assembly board, so that the assembly board cools quickly, and does not exist at the same time The phenomenon that the internal thermal energy lines remain in the board. For slow cooling, the residual heat energy in the assembly plate will be released to the environment, and compared with the rapid cooling, the assembly plate and seemingly cooled components will continue to maintain high temperatures for a period of time. Although the T between the two curves is only 1°C, it also has a certain impact on the lead-free process window of the PCB assembly.

In addition, it should be noted that rapid cooling will increase the internal stress of the solder joints, which may cause cracks in the solder joints of the SMT chip and component cracks. Due to various materials (different solders, PCB materials, Cu, Ni, Fe-Ni alloys) during the soldering process, the coefficient of thermal expansion (CTE) or thermal properties vary greatly, for example, the CTE of Sn-Ag-Cu is 15.5～17.1* 10 minus sixth power/ degree Celsius, Sn-Pb CTE is 21ppm/ degree Celsius, ceramic CTE is 5ppm/ degree Celsius, PCB material FR-4 horizontal direction CTE is 11～15*10 minus sixth power/ degree Celsius, The CTE in the vertical direction is 60-80 ppm/°C, and the CTE of the epoxy resin is also 60-80 ppm/°C. Therefore, when the solder joint is solidified, due to the cracking of related materials, the PCB metallization hole plating fractures and other welding defects. The cooling rate of Sn-Ag-Cu alloy from peak temperature to freezing point (245～217 degree Celsius) is generally controlled at -2～-6 degree Celsius/s.

(2) From close to below the freezing point (freezing point) of the solder alloy to 100°C.

It takes too long from the solder alloy wire (the freezing point of Sn-Ag-Cu alloy is 216 degree Celsius) to 100 degree Celsius. On the one hand, it will increase the thickness of IMC. Bi-coated lead-free components) may segregate due to the formation of dendrites, which may easily lead to solder joint peeling defects. In order to avoid the formation of dendrites, the cooling should be accelerated, and the cooling rate at 216～100 degree Celsius is generally controlled at -2～-4 degree Celsius/s.

(3) The outlet of the reflow oven is 100°C.

The main consideration is to protect the operators, and the outlet temperature is generally required to be lower than 60°C. Different ovens have different outlet temperatures. The nt outlet temperature of the equipment with high cooling rate and long cooling area is lower. In addition, theorists believe that the thickness of IMC will increase during the aging process of lead-free solder joints. Therefore, if it takes too long from 100°C to the outlet of the reflow oven, the thickness of the IMC will increase slightly.

In short, the cooling rate has an important influence on the quality of PCB assembly welding. Due to the internal microstructure of solder joints and defects in solder joints, components and printed boards, they cannot be detected from visual inspection. This will affect the long-term reliability of electronic products. Therefore, it is very important to control cooling; especially for amorphous lead-free solder, the cooling rate should be strictly controlled. This is the whole content of PCB assembly welding cooling process analysis, thanks for reading.