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The process of SMT patch processing mainly includes solder paste printing, placement, reflow soldering and AOI inspection. The process is more complicated. Irregular operations in any link will seriously affect the quality of SMT patch. Among them, the quality of the SMT patch can be controlled through the key links of steel mesh production, solder paste control, furnace temperature curve setting and AOI detection, so that the quality of welding can be effectively improved.
1. Steel mesh production
The solder paste is printed on the corresponding pads of the PCB through the stencil. The quality of the stencil has a very important influence on the soldering effect of the PCB pads. Among them, there are many defects such as more tin and less tin and the opening of the stencil. Big relationship.
The opening of the BGA steel mesh needs to be adjusted reasonably according to the size and gap of the BGA chip pin ball, and the adjustment is adjusted between the virtual soldering and the short circuit.
This is a reasonable value. There are many BGA package specifications in the product, which must refer to the chip situation and cannot be handled according to the general situation (recommended opening ratio of 88%-95%).
Attention should be paid to the following matters during the acceptance of steel mesh:
1. Check whether the method and size of the steel mesh opening meet the requirements.
2. Check whether the thickness of the steel mesh meets the product requirements.
3. Check whether the frame size of the steel mesh is correct.
4. Check whether the marking of the steel mesh is complete.
5. Check whether the flatness of the steel mesh is level.
6. Check whether the tension of the steel mesh is OK.
7. Check whether the position and number of openings in the steel mesh are consistent with the GERBER file.
A good stencil can miss good solder paste, laying a good foundation for improving the quality of subsequent soldering.
Second, solder paste control
Solder paste is used as a soldering material for SMT chip processing. The quality of solder paste has an important influence on the final soldering quality, and the solder paste needs to be strictly controlled.
1. Storage of solder paste
(1) The storage temperature of the solder paste is 0~10 degree Celsius. If it exceeds the storage temperature range, the temperature range of the refrigerator needs to be adjusted.
(2) The service life of solder paste is 6 months (unopened).
(3) The solder paste taken out of the refrigerator should not be placed in a place exposed to sunlight.
2. Use of solder paste
(1) The temperature of the solder paste must be raised to the ambient temperature of use (25±2 degree Celsius) before opening, and the temperature recovery time is about 3-4 hours, and it is forbidden to use other heaters to make the temperature rise instantaneously; Stir thoroughly. The mixing time of the mixer is 1-3 minutes, depending on the type of the mixer.
(2) Depending on the production speed, the amount of solder paste on the stencil should be added in small amounts and multiple times to maintain the quality of the solder paste.
(3) Solder paste that has not been used up that day should not be placed together with unused solder paste, and should be stored in another container. After opening the solder paste, it is recommended to use it up within 24 hours at room temperature.
(4) When using the next day, you should first use the newly opened solder paste, and mix the unused solder paste and the new solder paste at a ratio of 1:2, and add them in a small amount for multiple times.
(5) If the wire is changed for more than 1 hour, please scrape the solder paste from the steel plate and put it into the solder paste tank for sealing before changing the wire.
(6) After the solder paste is continuously printed for 24 hours, due to air dust and other pollution, in order to ensure the product quality, please follow the "step 4)" method.
(7) Please control the indoor temperature to 22-28 degree Celsius, humidity RH30-60% is the best working environment.
Three, reflow oven temperature curve setting
The setting of reflow soldering parameters is the key to the quality of soldering. The temperature curve can provide an accurate theoretical basis for the setting of reflow oven parameters. Each product has a corresponding temperature curve. When reflow soldering a new product, it is necessary to re-use the furnace temperature tester for testing.
The key places that affect the furnace temperature:
1. The temperature setting value of each temperature zone.
2. The temperature difference of each heating motor.
3. The speed of the chain and mesh belt.
4. The composition of solder paste.
5. The thickness of the PCB circuit board and the size and density of the components.
6. The number of heating zones and the length of reflow soldering.
7. The effective length and cooling characteristics of the heating zone.
Only a good furnace temperature curve can set the welding parameters in line with the product and improve the quality of reflow soldering.
Four, AOI detection
AOI detectors are often placed behind the reflow soldering process. AOI can detect many undesirable defects in the previous process, such as more tin, less tin, polarity direction, tombstone and other defects. Through AOI inspection, the problematic PCB board can be detected, avoiding the problematic board flow in the subsequent process, which is a very important link to improve the quality of SMT placement.
In the process of SMT patch processing, there are many factors that affect product quality. By strictly checking the above-mentioned key points, the quality of SMT patch can be effectively controlled.
