Factors affecting the quality of re - flow welding(3)

- Aug 28, 2018-

(6) reflow temperature curve

Temperature curve is the key to guarantee welding quality. The temperature rise slope and peak temperature of real-time temperature curve and paste temperature curve should be basic.Before the 160 ℃ heating speed control in 1 ℃ / s - 2 ℃ / s.If the heating rate is too high, on the one hand, the components and PCB will be subjected to too much heat, which will easily damage the components and cause PCB deformation.On the other hand, the solvent in the solder paste evaporates too quickly, easily splashes out metal components, producing tin beads.Peak temperature setting in general higher than metal melting point solder paste 30 ℃ to 40 ℃ or so (for example 63 sn / 37 pb solder paste melting point is 183 ℃, peak temperature should be set at 215 ℃ or so), then flow time for 30 s to 60 s.If the peak temperature is low or the reflux time is short, the welding will not be sufficient and the solder paste will not be melted seriously.If the peak temperature is too high or the reflux time is too long, the metal powder will be oxidized, the welding quality will be affected, and even the components and the board will be damaged.

Basis for setting reflow temperature curve:

1. Set according to the temperature curve of the solder paste.The welding paste with different metal content has different temperature curves. The reflow welding temperature curves of specific products should be set according to the temperature curve provided by the welding paste factory.

2. Set according to the material, thickness, whether it is multi-layer board and size of PCB board.

3. Set according to the density of components on the surface assembly board, the size of components and the presence of BGA, CSP and other special components.

4. According to the specific conditions of equipment, such as length of heating zone, material of heating source, construction of reflow welding furnace and heat conduction mode, etc.

There is a great difference between thermal stove and infrared furnace. Infrared furnace is mainly radiation conduction. Its advantages are high thermal efficiency, high temperature gradient, easy to control temperature curve, and easy to control the temperature on and down of PCB during double-sided welding.The drawback is that the temperature is not uniform.The temperature of the device varies with the color and size of the device on the same PCB.The welding temperature must be raised in order to bring the welding spot and large volume components around the dark color devices to the welding temperature.

The heat furnace is mainly convection.Its advantages are uniform temperature and good welding quality.The disadvantage is that the temperature difference between upper and lower PCB and the temperature gradient along the length of welding furnace are not easy to control.At present, many hot blast furnaces have adopted some improvement measures in the convection mode, such as small convection mode, independent air volume control in each temperature zone, and cooling method under the furnace to ensure the temperature gradient in the direction of upper, lower and length.To meet the requirements of the process curve.

5. Also according to the actual location of the temperature sensor to determine the temperature regions set temperature, if the temperature sensor location within the heating element, set the temperature higher than the actual temperature nearly, times the left and right sides, if the temperature sensor location in the top or bottom of the furnace chamber, set the temperature higher than the actual temperature 30 ℃ or so.

6. It is also set according to the size of the exhaust air volume. Generally, the reflow welding furnace has specific requirements on the exhaust air volume, but the actual exhaust air volume sometimes changes due to various reasons 

When determining the temperature curve of a product, take into account the amount of exhaust air and measure it regularly.

7. The ambient temperature also has an impact on the furnace temperature, especially the reflow welding furnace with short heating temperature zone and narrow width of furnace body. The furnace temperature is greatly affected by the ambient temperature.

(7) quality of reflow welding equipment

The quality of re - flow welding is closely related to the equipment.Main parameters affecting reflow welding quality:

1. The temperature control precision, with up to + / - 0.1-0.2 ℃ (temperature sensor sensitivity to meet the requirements).

2. The conveyor belt transverse temperature + 5 ℃ below, otherwise it is difficult to ensure the welding quality.

3. The width of conveyor belt should meet the requirement of maximum PCB size.

4. Length of heating zone -- the longer the heating zone and the more heating zone, the easier it is to adjust and control the temperature curve.Select 4-5 temperature zone for medium and small batch production, and the heating zone length is about 1.8m, which can meet the requirements.In addition, the upper and lower heaters should be independently controlled to adjust and control the temperature curve.

5. The highest heating temperature is 300-350 ℃, if consider lead-free solder or metal substrate, should choose above 350 ℃.

6. The conveyor belt running smoothly, conveyor belt, vibrations will cause the shift, can be seen from the above analysis, solder reflow soldering quality and PCB design, component solderability, solder paste quality, processing quality of printed circuit boards, production line equipment, and SMT process parameters of every work, even with the operation of the operating personnel have close relationship.At the same time, it can be seen that PCB design, PCB processing quality, components and solder paste quality are the basis to guarantee the quality of reflow welding, because these problems are difficult or even impossible to be solved in the production process.