The three most important steps in the SMT process

The purpose of this paper is to describe the

There are three main steps in the SMT process: solder paste printing, component placement and reflow soldering. The first thing to do after soldering.

“Visual inspection”, this method can also be regarded as a way to judge whether the manufacturing process is good or bad after the process step.

2. Solder paste printing

First check whether the parameter settings of the solder paste printing machine are correct. The solder paste of the board should be on the pad, whether the height of the solder paste is consistent or in a “trapezoidal” shape, and the edges of the solder paste should not have rounded corners or collapse into a pile. shape, but it is allowed to have some peak shape caused by pulling up some solder paste when the laser stencil is separated. If the solder paste is not evenly distributed, it is necessary to check whether the solder paste on the scraper is insufficient or unevenly distributed. Need to check the printing SMT stencil and other parameters. Finally, the solder paste should be shiny or wet rather than dry under the microscope.

3. Component placement

Before placing components on the first board with solder paste, you should first confirm whether the material rack is properly placed, whether the components are correct, and whether the pick-and-place position of the machine is correct.

After completing the first board, check in detail whether each part is correctly placed and lightly pressed in the center of the solder paste, rather than just “placed” on top of the solder paste. If you can see the solder paste is slightly concave under the microscope, it means that the placement is correct. In this way, the phenomenon of “slipping” of components during reflow can be avoided.

Need to confirm again whether the surface of the solder paste is still wet? If the board has been printed with solder paste for a long time, the solder paste will appear to have a dry and cracked surface.

Such solder paste will cause “rosin solder joints”, which cannot be detected except after passing through the soldering oven. This type of rosin solder joint is usually found in the through-hole assembly process, which will cause a thin transparent rosin layer between the component and the pad, and block any electrical transmission.

last second check

l Are all the components on the bom consistent with the components on the board?

l Are all components sensitive to positive and negative poles such as diodes, capacitors and ic parts placed in the correct direction?

Four, through the welding furnace

Once the over-soldering temperature curve is set (that is to say, many boards have been measured with thermocouples in advance and it is determined that there are no defects), only when there is a large change in the number or a major missing occurs, the over-soldering will be adjusted in line. curve. The so-called “perfect” solder joints are bright and smooth in appearance and have complete solder coating around the pins.

Some oxides mixed with rosin residues can also be seen near the solder joints, which indicates that the flux has a cleaning function. This oxide is normal and usually comes off the PCB, but is more likely to come off the pins on the component due to the cleaning action of the flux, which also indicates that the component may have been stored for a while A long time, even longer than pcb.

Old or incompletely mixed solder paste may produce small solder balls due to poor welding conditions with pads or component pins (Note: small solder balls may also be caused by process defects such as moisture in the solder paste Or caused by defective green paint). However, the poor welding condition may also be due to poor management, so that some boards have been touched by the staff, and the grease on the hands remains on the pad to cause the defect. Of course, this phenomenon may also be caused by too thin tin plating on the pad or component feet.

Finally, to an inspector, grayish solder joints can be the result of too old solder paste, too low a reflow temperature, too short a reflow time, or an incorrectly set reflow profile, or a reflow process. The furnace is malfunctioning. The small solder balls may be caused by the fact that the board has not been baked or it has been baked for too long, or the components are too hot or the components are placed. Someone adjusted the components before entering the soldering furnace, and the solder paste was squeezed out. caused by out-of-pad.

5. Visual inspection skills

This method can be used to inspect boards that have just passed through a soldering oven. First scan the entire board with eyes, and then use a microscope to check for defects. Such as lack of tin, short circuit or twisted pins can be easily found by tilting the board to adjust the best line of sight. It is often easier and less time-consuming to inspect irregularities by eye than to inspect bit by bit with a microscope. When a problem is discovered, a microscope is used for more detailed inspection. Continuous practice and memorizing the position and appearance of components on the board can effectively improve the inspection speed and the detection rate of defects.

For small components, the same method can be used. Most of the small components will be 90° or parallel to the edge of the board, so as long as the soldering is incomplete, most of the small components will not be in contact with the board. The sides are parallel or perpendicular, but appear to be skewed from the pad, tombstoning, or shorted. One of the reasons is that one pad is heated and welded to the pin faster than the other pad, causing tin to flow and weld to this pad, and the part is pulled to this pad due to the cohesive force of tin itself , so that the tin on the other pad has no time to fuse with the pin, and the part “stands up”. Another reason is caused by defects in the wiring of the pads and the pins of the components themselves.

In addition, almost without exception, the part legs of IC components are bent, either causing empty soldering or poor soldering.

Distinguish quality by color

The microscope should only be used to inspect defects at the first site. From the color or from the brightness (or particles) of the hardened rosin and solder paste surface, it can be distinguished whether the maximum temperature during over-soldering is sufficient, too low or too high . The rosin spots responsible for the insulation of the pads and contacts were also discerned by microscopy.

The ideal color of rosin should be transparent or transparent with white. If it is slightly yellow, it means that the welding temperature is too high.

If the joints of the component pins and pads on the board are wrinkled rather than smooth, it means that the temperature of the soldering furnace is too high or the cooling is too fast. The over-soldering curve when assembling the board must be carefully measured by thermocouples, and the best over-soldering curve can be obtained before this curve can be applied to the official product.

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