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Analysis of smt defect list

https://www.pcbasic.com/smt_assembly_test.htmlThis is a smt defect list. We will tell you 4 common SMT defects and factors in this article. 


The following is the smt defect list.
1. Causes and treatment of solder beads

This is one of the most common smt defects, mainly around the chip resistance-capacitance component (CHIP), and is caused by many factors.

Factor 1: The selection of solder paste directly affects the quality of soldering

The factors that cause this smt defect include the metal content in the solder paste, the degree of oxidation of the solder paste, the particle size of the alloy solder powder in the solder paste, etc. Before using the solder paste, it is generally refrigerated in the refrigerator. After taking it out, it should be restored to room temperature and then opened for use. Otherwise, the solder paste will easily absorb moisture and cause solder splashes when the reflow solder splashes. Therefore, the selection and correct use of solder paste brands directly affect the production of solder balls.

Factor 2: Production and opening of steel plate (template)

When printing solder paste, it is easy to print the solder paste on the solder resist layer so that solder beads are generated during reflow soldering. The shape of the opening can be changed to achieve the desired effect. At the same time, too thick solder paste will cause the solder paste to "collapse" and promote the generation of solder beads.

Factor 3: placement pressure of the placement machine

Excessive patch pressure can also cause this smt defect. Suppose the pressure is too high during placement. In that case, the solder paste will easily be squeezed onto the solder mask under the component, and the solder paste will melt and run around the component to form tin beads during reflow soldering. The solution can reduce the pressure during mounting and adopt a suitable steel plate opening form to prevent the solder paste from squeezing the pad.

Factor 4: Setting of the furnace temperature curve

Tin beads are generated when the printed board passes through reflow soldering. The flux in the solder paste vaporizes, which may cause small particles of metal to separate and run under the components and then run around the components to form tin beads during reflow. At this stage, the temperature should not rise too fast. Too fast can easily cause solder splash and form tin beads. Therefore, the temperature curve of reflow soldering should be adjusted, and a moderate preheating temperature and preheating speed should be adopted to control the production of tin beads. Just like this, we will explain the possible reasons for each situation in detail based on the smt defect list.

2. Analysis and treatment of tombstone problem

The tombstone problem is the second common smt defect. One end of the rectangular chip component is soldered to the pad, while the other is upright. This phenomenon is called a tombstone. The main cause of this phenomenon is the uneven force on both ends of the component when the solder paste melts. So what are the factors that cause this smt defect? Take a look at our conclusions based on the smt defect list.

Factor 1: uneven thermal efficiency and different melting rates of solder joints

Factor 2: Uneven solderability of the two solder ends of the component or the two points of the PCB pad

Factor 3: The deviation is too large when mounting the component, or the connection surface of the solder paste and the component is too small

In view of the above factors, the following methods can be used to reduce the tombstone problem:

1. Appropriately increase the temperature of the reflow curve

2. Strictly control the solderability of circuit boards and components

3. Strictly keep the solder paste thickness of each soldering corner consistent

4. Avoid major changes in the environment

5. Control the offset of components during reflow

6. Increase the pressure between the corner of the component and the solder paste on the pad

Third, the bridging problem.

The bridging problem is the third smt defect we want to talk about. There is a solder connection between the solder joints causing a short circuit. There are 6 reasons according to smt defect list.

cause:

1. Due to the small deviation between the stencil opening and the pad, the solder paste printing is not good, and there is a deviation

2. Too much solder paste may be due to the excessive proportion of stencil openings

3. Solder paste collapse

4. The shape of the solder paste after printing is not good, and the molding is poor

5. Reflow time is too slow

6. The contact pressure between the components and the solder paste is too large


Correspondingly, the smt defect list gives 6 solutions.


Solution:

1. Choose a solder paste with a relatively high viscosity. Generally speaking, there are more bridging phenomena between 85 and 87%, and at least the alloy content should be above 90%.

2. Adjust the appropriate temperature curve

3. Before reflow soldering, check whether the contact points of the solder paste and the device are suitable

4. Adjust the ratio of steel mesh openings (10% reduction) and the thickness of the steel mesh

5. Adjust the pressure and angle of the patch

Fourth, the bad phenomenon of incoming solder rejection
The fourth common smt defect: incoming solder rejection. There are generally four reasons based on smt defect list.

1. Rejection of parts

Phenomenon characteristics: all the metal tin stays on the surface of the PCB PAD, forming an arched surface. There is no metal tin climbing on the part eating tin surface.

According to the reasons that cause the parts to be rejected, they are divided into:

(a) Part solder rejection phenomenon caused by the oxidation of the part's tin surface

(b) Part body manufacturing industry causes part solder rejection phenomenon

2. PCB PAD solder rejection phenomenon

Phenomenon feature: all-metal tin climbs to the surface of the part to eat the tin and form an arched surface, but there is no metal tin on the surface of the PCB PAD

The main reason: the solder rejection phenomenon caused by the oxidation of the tin surface of the parts

3. The phenomenon of empty welding caused by parts lifting

Phenomenon characteristics: the parts feet have different degrees of warping, the metal tin is evenly distributed on the PCB PAD and forms a smooth luster

Two manifestations:

(a) The part feet are partially tilted and are in the shape of a moon

(b) The whole part of the foot is tilted parallel to the tin surface

4. The phenomenon of empty soldering caused by foreign matter and less tin


After reading this smt defect list, do you have a general grasp of common smt defects? There are many other common SMT defects. If you want to know more about SMT defects, please follow us. More information: smt defects types, smt defects' influence, and smt defects solutions.

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