What is SMT (Surface Mount Technology) or SMD (Surface Mount Device)?
SMD: Surface Mount Device, refers to components that are surface-mounted or soldered onto the surface of a circuit board. So, this term denotes components that can be used with SMT processes or technology in electronics.
SMT: Surface Mount Technology, refers to a technique or technology for soldering electronic components onto the surface of a circuit board. So, this term refers to a method of soldering electronic components onto the surface of a circuit board.
What Factors and Considerations Affecting the Quality of SMT Solder Paste Printing?
Solder paste printing is the foundation of the soldering quality on a printed circuit board, with the position and quantity of solder paste being crucial. It is common to encounter issues such as solder shorts and solder empties when the solder paste printing is not done properly. However, to achieve good solder paste printing, the following factors must be taken into consideration:
The selection of an appropriate squeegee for solder paste printing should be based on the characteristics of different solder pastes or red adhesives. Currently, squeegees used in solder paste printing are typically made of stainless steel.
The angle at which the squeegee scrapes the solder paste is generally between 45 to 60 degrees.
The pressure applied by the squeegee affects the volume of solder paste. In general, under unchanged conditions, higher squeegee pressure results in less solder paste. This is because higher pressure compresses the gap between the steel stencil and the circuit board.
- Squeegee Speed:
The speed of the squeegee also directly influences the shape and volume of the solder paste printing, and it significantly affects the quality of the solder paste printing.
The squeegee speed is usually set between 20 to 80 mm/s. In principle, the squeegee speed must be matched with the viscosity of the solder paste. Solder pastes with better flowability require faster squeegee speeds; otherwise, there may be issues with solder bleeding. In general, a faster squeegee speed results in less solder paste being applied.
Steel Stencil Peeling Speed:
If the peeling speed is too fast, it may lead to phenomena such as solder paste dragging or spiking, potentially affecting the placement results.
Use of Vacuum Block: Whether to use a vacuum block? A vacuum block can help the circuit board adhere smoothly to a fixed position, enhancing the tightness between the steel stencil and the circuit board. Sometimes, for products produced in small quantities or for one-time use, universal top pins/blocks can be used as an alternative to vacuum blocks.
Circuit Board Warpage: Is the circuit board warped? Warped circuit boards can result in uneven solder paste printing, often causing short circuits in most cases.
The openings in the steel stencil directly impact the quality of solder paste printing. The cleanliness of the steel stencil is crucial, and it directly relates to the quality of solder paste printing, especially on the contact surface between the steel stencil and the PCB. This is to prevent residual solder paste under the steel stencil from contaminating positions on the PCB where solder paste should not be present. In general, SMT manufacturing facilities often specify the use of lint-free wiping paper to clean the bottom of the steel stencil after producing a certain number of boards. Some even design automatic wiping functions on the printing machine. Additionally, there are guidelines for periodically removing the steel stencil, using solvents and ultrasonic vibrations for cleaning. The purpose is to remove any remaining solder paste in the stencil apertures, especially for fine-pitch components, ensuring that solder paste printing is not obstructed.
What Should the SMT CPU SOCKET be baked before assembly, and what temperature and duration should be set?
I have not baked it before because its TRAY tends to deform, but recently, there have been many occurrences of solder voids, prompting me to consider baking it.
Based on my experience, a CPU SOCKET should not require baking. If baking is deemed necessary, it may be due to issues with the plastic used by the manufacturer. Additionally, not baking before assembly may lead to OPEN issues.
I speculate on several potential problems:
1. Issues with the plastic
2. PCB bending problems (for boards thinner than 0.8mm)
3. Component flatness issues
4. Component moisture absorption issues (Is there vacuum control? J-STD-033A defines this)
When the CPU goes through the reflow oven, does the temperature reach 220 degrees Celsius (the actual melting point of solder) for 20-60 seconds? If so, and solder still has voids, attention should be given to:
1. PCB warping or CPU warping issues
2. Wetting issues between solder paste and solder balls (related to oven uniformity)
3. Surface treatment of solder balls and PCB pads. If the solder did not melt, consider:
- Temperature measurement problems, insufficient temperature, and duration
- Issues with solder paste (storage, flux, and oxidation)
Note: The provided translation is a direct conversion of the text and may require further refinement based on the context and specific technical terms used in your industry.
If you are an SMT engineer, you must have used X-Ray to inspect the soldering conditions of BGA. However, when you look at it, all the solder balls of the BGA seem to look the same. How do you determine if there are solder voids in the BGA?
In my experience, most people using X-Ray can generally only check for issues such as short circuits, insufficient solder, and voids. However, using X-Ray to determine if there are solder voids in BGA solder balls can be challenging, especially with 2D X-Ray. Of course, if you are careful, you can still find some subtle signs to judge whether there are solder voids in BGA solder balls.
Generally, X-Ray images are simple 2D projection views. It is easy to use them to check for short circuits, but using them to check for solder voids may be challenging for many people. This is because the solder balls of each BGA look almost round, making it difficult to discern whether there are solder voids. Although in recent years, there is the so-called “3D X-Ray CT” that claims to provide 3D images, the cost of taking such images can be prohibitive.
BGA Solder Ball Enlargement Leading to Solder Voids
First, consider that the solder balls of the same BGA should be of the same size. Suppose there are some solder balls with voids in the same BGA, while most solder balls do not have voids. Do you think these two types of solder shapes should be somewhat different? The answer is affirmative. Consider that when compressed, solder with no voids should disperse some of the solder onto the PCB pad, reducing the solder ball diameter. However, solder balls with voids will not exhibit this behavior; instead, compression may lead to an increase in the solder ball diameter.
Solder Voids Due to Insufficient Solder Volume from Vias
Another type of BGA solder void phenomenon is caused by insufficient solder volume. This situation often occurs when there are vias in the solder pads. During the reflow soldering process, some solder may flow into the vias due to capillary action (wicking), leading to insufficient solder volume. Sometimes, vias near the solder pad can also cause this problem. In such cases, the solder balls seen on X-Ray images will appear smaller, and if too much solder is drawn into the vias, it can result in solder voids. Generally, it is not recommended to place vias on solder pads, and vias near solder pads should be covered with green solder mask.
Solder Voids Due to Internal Bubbles in Solder Balls
Another cause of BGA solder voids is the presence of voids (bubbles) within the solder balls. According to the specifications of IPC7095 126.96.36.199, which is commonly used in the electronics industry for Class II applications, the combined diameter of all bubbles should not exceed 60% of the BGA diameter. If the bubbles are too large, it can lead to solder voids or solder fractures.