PCB Assembly Manufacturing SMT Quality Standards
Ⅰ. Terms for SMT quality
1. Ideal solder joints
It has good surface wettability, that is, the molten solder should spread on the surface of the welded metal and form a complete, uniform and continuous solder covering layer, and its contact angle should not be greater than 90.
The correct amount of solder, the amount of solder is sufficient but not too much or too little.
For a good welding surface, the surface of the solder joint should be complete, continuous and smooth, but it does not require a very bright appearance.
For good solder joints, the position deviation of the solder ends or pins of the components on the pads is within the specified range.
2. No wetting
The contact angle formed by the solder on the solder joint and the surface of the metal to be soldered is greater than 90.
3. Open welding
The pad is separated from the PCB surface after soldering.
4. drawbridging
One end of the component leaves the pad and faces the bag upright diagonally or upright.
5. Bridging
The solder between two or more solder joints that should not be connected is connected, or the solder joints of the solder joints are connected with adjacent wires.
6. Welding
After soldering, electrical isolation sometimes occurs between the solder end or the lead and the pad.
7. Draw the tip
There are burrs protruding from the solder in the solder joints, but they are not in contact with other conductors or solder joints.
8. Solder ball
A small ball of solder that adheres to a printed board, solder mask, or conductor during soldering.
9. Holes
There are cavities with different apertures in the weld.
10. Skewing
When the solder joint deviates from the predetermined position in the horizontal, vertical or rotational direction in the plane.
11. Visual inspection
With the aid of a magnifying glass of 2~5 times of the illumination, the quality of PCBA solder joints can be inspected with naked eyes.
12. Inspection after aoldering
Quality inspection after PCB is soldered.
13. Reworking
Repair process to remove local defects of surface mount components.
14. Placement inspection
When surface mount components are mounted or after they are mounted, quality inspection is carried out for any missing, misplaced, mis-mounted, damaged, etc. conditions.
Ⅱ. Inspection methods of SMT
In SMT inspection, two methods, visual inspection and optical equipment inspection, are often used. Some only use visual inspection, and there are also two mixed methods. All of them can inspect 100% of the products, but people will always get tired if visual inspection is used, so that 100% of employees cannot be guaranteed to inspect carefully. Therefore, we have to establish a balanced inspection and monitoring strategy that is to establish quality process control points.
In order to ensure the normal operation of SMT equipment, strengthen the quality inspection of the processed workpieces in each process, so as to monitor its operating status, and establish quality control points after some key processes. These control points are usually set up in the following locations:
1) PCB inspection
a. Whether the printed board is deformed; b. Whether the pad is oxidized; c. Whether the surface of the printed board is scratched;
Inspection method: visual inspection according to inspection standards.
2) Screen printing inspection
a. Whether the printing is complete; b. Whether there is a bridge; c. Whether the thickness is uniform; d. Whether there is sag; e. Whether there is any deviation in printing;
Inspection method: visual inspection according to inspection standards or inspection with magnifying glass.
3) Patch inspection
a. Placement of components; b. Whether there is a film drop; c. Whether there are any wrong parts;
Inspection method: visual inspection according to inspection standards or inspection with magnifying glass.
4) Reflow soldering inspection
a. The soldering condition of the components, whether there are bad soldering phenomena such as bridging, tombstones, misalignment, solder balls, and virtual soldering. b. Solder joints.
Inspection method: visual inspection according to inspection standards or inspection with magnifying glass.
Ⅲ. Guidelines for inspection standards
l Printing inspection
General rule: The amount of solder paste printed on the pads is allowed to have a certain deviation, but the area covered by the solder paste on each pad should be greater than 75% of the area of the pad.
l Dispensing inspection
Ideal glue point: candle = no traces of SMD glue contamination on the pad and lead-out end surface. The glue dot is located in the middle of each pad, and its size is about 1.5 times that of the glue nozzle. The amount of glue is based on the solder end of the component after mounting It is advisable not to occupy the dirt with the PCB pad.
l Pre-furnace inspection
l Post-furnace inspection
A good solder joint should be full, well wetted, and solder spread to the edge of the pad.
Ⅳ. Statistics of the number of quality defects
In the SMT production process, the statistics of quality defects are very necessary. In the quality defect statistics of reflow soldering, we have introduced the DPM statistical method, that is, the defect statistical method of parts per million. Calculated as follows:
Defect rate [DPM]=total number of defects/total number of solder joints*106
Total number of solder joints = number of test circuit boards × solder joints
The total number of defects = the total number of defects of the tested circuit board
For example, there are a total of 1000 solder joints on a circuit board, the number of detected circuit boards is 500, and the total number of defects detected is 20, it can be calculated according to the above formula:
Defect rate [PPM]=20/(1000*50)*106=40PPM
Ⅴ. Repair
After the PCBA inspection is completed, the defective PCBA needs to be repaired. The company has two methods for repairing the SMT PCBA. One is to use a constant temperature soldering iron (manual welding) for repair, and the other is to use a repair workbench (hot air welding) for repair. No matter which method is adopted, it is required to form a good solder joint in the shortest time. Therefore, when using a soldering iron, it is required to complete the soldering point in less than 5 seconds, preferably about 3 seconds.
A. Ferrochrome rework method is manual welding
Treatment of the new soldering iron before use: The new soldering iron can be used normally after the soldering iron tip is plated with a layer of solder before use. When the soldering iron is used for a period of time, an oxide layer will be formed on the blade surface of the soldering iron tip and the surrounding area. If it is difficult to "eat tin", you can file off the oxide layer and re-plated solder.
Holding method of electric soldering iron:
a. Reverse grip: Use five fingers to hold the handle of the soldering iron in your palm. This method is suitable for high-power electric soldering irons to weld the welded parts with large heat dissipation.
b. Ortho-grip method: Hold the handle of the soldering iron with four fingers except the thumb, and press the thumb along the direction of the soldering iron. The soldering iron used in this method is also relatively large, and most of them are curved soldering iron tips.
c. Pen holding method: holding an electric soldering iron like holding a pen, suitable for low-power electric soldering irons to weld small parts to be welded. The company uses the pen-holding method.
Precautions for welding:
1. The temperature of the soldering iron tip should be appropriate. Putting the soldering iron tip of different temperature on the rosin block will produce different phenomena. Generally speaking, the temperature when the rosin melts faster and does not emit smoke is more suitable.
2. The soldering time should be appropriate, from heating the soldering point to the melting of the solder and filling the soldering point, it should generally be completed within a few seconds. If the soldering time is too long, the flux on the soldering point will be completely volatilized, and the fluxing effect will be lost. If the soldering time is too short, the temperature of the soldering point will not reach the soldering temperature, and the solder will not melt sufficiently, which will easily cause false soldering.
3. Appropriate amount of solder and flux should be used. In general, excessive or too little use of solder and flux on solder joints will have a great impact on soldering quality.
4. Prevent the solder on the solder joints from flowing randomly. The ideal soldering should be that the solder is only soldered where it needs to be soldered. In the soldering operation, the solder should be less at the beginning. When the soldering point reaches the soldering temperature, the solder will flow into the gap of the soldering point and then refill the solder to complete the soldering quickly.
5. Do not touch the solder joints during the soldering process. When the solder on the solder joints has not completely solidified, the soldered devices and wires on the solder joints should not be moved, otherwise the solder joints will be deformed and virtual welding will occur.
6. Don't scald the surrounding components and wires. When soldering, be careful not to scald the plastic insulation layer of the surrounding wires and the surface of the components, especially the products with compact welding structure and complicated shape.
7. Do the cleaning work after welding in time. After the welding is completed, the cut wire ends and the tin slag dropped during welding should be removed in time to prevent hidden dangers from falling into the product.
B. Rework of the rework workbench
The use of the rework workbench is mainly used when the defects of QFP, BGA, PLCC and other components cannot be reworked manually. It usually uses hot air heating to heat the solder feet of the components, but it must be matched with the corresponding nozzle. The higher-level rework workbench can have a temperature profile similar to that of the reflow furnace in the heating zone, such as the company's SMD-1000 rework workbench. For the operation of the rework workbench, please refer to the manual.
