PCBA Tech - smt assembly welding process and qualification conditions

Chip components are often called Chip components in smt mounting technology. They are generally rectangular chip resistors, capacitors and other passive components. Their structural appearance no longer has pins protruding, but has two terminal poles on both sides of the rectangle, which are used for welding. The soldered area of the two end poles, the position on the pad, the climbing height of the solder on the end pole, etc. are all required by the smt process in assembly and soldering. Next, let's explain and analyze.

1. Mounting requirements and qualification judgment of chip components

Before the chip components are soldered, how their mounting position is directly related to the success or failure of the subsequent soldering process. Therefore, the assembly and welding of rectangular chip components often use equipment placement, manual welding, etc. for assembly and welding. The requirements and judgments are as follows.

1. The soldering end faces of chip components are required to be all located on the pads and are required to be centered. This mounting position is the best, the most excellent, and the recommended mounting position.

2. When mounting, when there is a deviation in the up and down direction, it is required that the soldering end of the chip component and more than two-thirds of the width should be on the pad. There must be such a required mounting area to be considered qualified ( Only applicable when solder mask is coated on the PCB).

3. If the soldering end of the chip component is less than two-thirds of its width on the pad (only one-third of the soldering end is on the pad), this type of mounting should be considered unqualified.

4. If the soldering terminal of the chip component overlaps with the land, the remaining part A of the land is not less than 1/3 of the height of the soldering terminal, which is regarded as qualified. This is to check the placement of chip components from the perspective of whether the placement is centered or not.

5. The soldering end of the chip component does not overlap with the pad (including the component soldering end just leaning on the edge of the pad), and the serious misalignment causes one side to deviate from the pad, and there is a distance from the pad, distance B ≥0, such placement is regarded as unqualified.

6. If the chip component has a rotation deviation in the up and down position of the pad, the rotation distance D is greater than or equal to two-thirds of the component width, which is considered qualified (when wave soldering is used, it should be considered unqualified). Otherwise it should be considered as unqualified

Second, the welding requirements and determination of chip components

The soldering requirements of smt chip components and the soldering requirements of THT plug-in components should be the same, because they are all soft soldering, and the composition and proportion of the alloy solder used are basically the same, so the solder joints The judgment is the same, just because of the difference in shape and structure. The first requirement is also wetting, followed by the amount of solder and the shape after soldering.

In general, the welding process requirements of rectangular chip elements should be: the welding end has good wetting, the solder joint is a meniscus, and the tin thickness between the pad and the terminal is about 0.05mm. It is a good solder joint.

1. The solder terminals of chip components have good wetting, the solder joints on the terminal surfaces are meniscus, and the amount of solder between the pad and the terminal is moderate. Such solder joints should be regarded as good solder joints.

2. The qualified condition of chip component welding is nothing more than the form and requirements described above, but the unqualified welding form will have many manifestations.

1) The solder end of the chip component is offset by one-half of the land.

2) The chip component has a rotation deviation at its end after soldering. If this rotation deviation exceeds half or more than half of the mounting pad, it should be regarded as unqualified.

3) The chip component is not in the center position of the pad after smt patch proofing or processing and welding, and it is translated out of the pad. If the translation of either side exceeds the pad, the terminal is less than half or less than half of the pad position, Should be regarded as unqualified.

4) The chip component is not at the position of the pad after soldering, and it is translated out of the pad, which should be regarded as unqualified soldering.

5) The solder at either end of the chip component climbs too high, the height "E" exceeds the soldering end, the appearance of the solder joint has no meniscus shape, and the solder extends to the top of the component metal plating end cap, although it has not yet touched The component body, but such a welding form should be regarded as unqualified.