SMT Soldering
With the development of economy and science and technology, people have put forward higher and higher requirements for electronic products with multi-function, miniaturization, high density, high performance and high quality. Therefore, the high soldering quality of SMT is the most important for electronic products.
However, in actual manufacturing, soldering defects often occur, especially during the reflow stage.
In fact, the soldering problems that occur at this stage are not entirely caused by the reflow technique, because the SMT soldering quality is closely related to the manufacturability of the PCB pads, the stencil design, the solderability of the components and the PCB pads, the state of the manufacturing equipment, the quality of the solder related. The technical parameters of each link and the operation of each engineer have an impact.
Problems can occur in every link of the process, which can affect the quality of SMT welding.
Quality of BOM
As one of the most important composite materials in SMT, the quality and performance of BOM is directly related to the quality of reflow soldering.
Specifically, the following aspects must be considered:
Component Packaging
Component packaging must meet the automatic installation requirements of the installation procedure.
Part Graphics
Part graphics must meet the requirements of automatic SMT because it must have standard shapes with high dimensional accuracy.
Welding Quality
The soldering quality of the solderable ends of the components and the PCB pads shall meet the requirements of reflow soldering, and the solderable ends of the components and the pads shall not be contaminated or oxidized.
If the solderable ends of components and PCB pads suffer from oxidation, contamination or moisture, some soldering defects such as poor wetting, false soldering, solder beads or cavities may occur. This is especially true for humidity sensors and PCB management.
The humidity sensor must be stored in a dry box after vacuum packaging, and it is necessary to bake it before the next manufacturing.
Design of PCB Pads
The level of SMT depends on the quality of the PCB design and is the first factor that affects the quality of the surface mount. According to statistics, 70% to 80% of manufacturing defects arise from PCB design issues in substrate material selection, conversion component layout, pad and thermal pad design, solder mask design, component package type, component method, transmission boundary, By positioning, optical positioning points, EMC, etc.
For a PCB with the correct pad design, even a slight skew during surface mounting can be corrected by the surface tension of the molten tin, which is called the auto-positioning or self-correcting effect.
However, if the PCB pads are not designed correctly, even if the mounting location is very accurate, soldering defects will still be encountered.
Such as component position offsets and tombstones. Therefore, the following aspects must be carefully considered in SMT pad design:
Symmetry of the pads. In order to avoid position shift and tombstone problems after reflow soldering, for 0805 and below chip components, the pads at both ends should be symmetrical in terms of pad size and heat absorption and heat dissipation capabilities to maintain a balance of melting surface tension. Soldering If one end is on the large copper foil, it is recommended to use a single wire connection to connect the pads on the large copper foil.
space between pads. Soldering defects tend to result when the space between the pads is too large or too small in order to ensure the proper size of the overlap between the end of the component or between the pins and the pads.
The remaining dimensions of the pad must ensure a meniscus solder joint between the end of the component or the lap between the lead and the pad.
The width of the pad should be roughly compatible with the width of the component end or pin.
Do not place vias on pads. Otherwise, during reflow soldering, molten tin may flow away along the vias, resulting in spurious soldering and tin starvation. It could flow to the other side of the board causing a short.
Solder Paste Printing
The main purpose of solder paste printing technology is to solve the problem of incompatibility with the amount of solder paste printing, which refers to the filling amount and transfer amount of solder paste.
According to professional statistics, 60% of reworked PCBs are caused by poor solder paste printing when PCBs are properly designed. In solder paste printing, three important “S”s must be remembered:
Solder paste, stencil and squeegee. If selected correctly, you can achieve excellent printing results.
Quality of Solder Paste
As a necessary material for reflow soldering, solder paste is a kind of solder paste that is uniformly mixed with alloy powder and flux (including rosin, thinner, stabilizer, etc.), in which alloy powder is the key element to form solder joints. Flux is a key material for eliminating surface oxidation, improving wetting and ensuring solder paste quality.
Generally speaking, 80% to 90% of solder paste belongs to metal alloy, and it accounts for 50% of its volume. Solder paste quality assurance mainly comes from two aspects:
Storage and Application, Solder paste is usually stored between 0 and 10°C, or according to the manufacturer’s requirements.
For its application, the temperature of the SMT workshop must be 25℃±3℃, and the humidity must be 50%±10%.
Recovery time must be 4 hours or more, and must be well agitated prior to use to give the viscosity excellent printability and release deformability. The solder paste cap must be placed correctly after the solder paste is applied, and the solder paste-coated circuit boards must be reflow soldered within two hours.
Stencil Design
The key function of the stencil is to apply solder paste evenly on the PCB pads. Stencil is essential in printing technology, and its quality directly affects the quality of solder paste printing. So far, there are three methods of making stencils:
Chemical etching, laser cutting and electroplating.
Mold design cannot be assured until the following aspects are fully considered and properly addressed.
Thickness of Steel Plate
In order to ensure the quantity and quality of solder paste, the surface of the stencil must be smooth and flat, and the choice of the thickness of the stencil should be determined by the component with the smallest spacing between pins.
Aperture Design
The opening is a trapezoidal section hole, and the opening is a trumpet shape. Their walls are smooth without burrs. Aspect ratio = width of hole / thickness of stencil (for small pitch QFP, IC)
Area ratio = base area of hole / wall area of hole (for 0201, BGA, CSP parts)
Solder Ball Processing
Solder ball resist treatment implemented on stencil holes of 0603 or above CHIP components can effectively avoid solder balls after reflow. For components with too large pads, meshing is recommended to prevent excessive tin generation. d The B side of the marked stencil should produce at least 3 MARK points, and the stencil should be compatible with the MARK on the PCB.
To increase printing accuracy, there should be a pair of MARK points with the longest diagonal distance. Printing direction is also a critical control point. Components with a small spacing from each other should not be too close to the rails during the process of orienting the print. Otherwise, too much tin can cause bridging.
Scraper Machine
Squeegees can affect print quality to some extent based on their different hardness materials and shapes. Typically, steel squeegees with nickel plating are used, typically 60° squeegees. If there are through-hole components, a 45° scraper is recommended to increase the tin content on the through-hole components.
Printing Parameters
Printing parameters mainly include squeegee speed, squeegee pressure, stencil downward release speed, stencil cleaning mode and frequency. There is indeed a limiting relationship between the angle of the squeegee and the stencil and the viscosity of the solder paste, so only by properly controlling these parameters can the printing quality of the solder paste be ensured.
In general, the low speed of the squeegee results in relatively high print quality and the shape of the solder paste may be blurred. Furthermore, extremely low speeds even reduce manufacturing efficiency. Conversely, a high-speed squeegee may cause underfilling of the solder paste in the mesh.
Too high a squeegee pressure can result in insufficient tin, increasing wear between the squeegee and stencil, while very low pressure can result in incomplete solder paste printing.
Therefore, when rolling solder paste normally, the speed should be as high as possible. Also, the squeegee pressure should be adjusted for high print quality. Extremely high down release speeds can cause paste freezing or poor formation, while slow release speeds can affect manufacturing efficiency.
Improper stencil cleaning method and frequency can result in incomplete stencil cleaning, continuous tin electrodeposition or insufficient tin in the stencil apertures resulting in narrow space products. Extremely high down release speeds can cause paste freezing or poor formation, while slow release speeds can affect manufacturing efficiency.
Improper stencil cleaning method and frequency will result in incomplete stencil cleaning, continuous tin electrodeposition or insufficient tin in the stencil holes will result in narrow space products.
Extremely high down release speeds can cause paste freezing or poor formation, while slow release speeds can affect manufacturing efficiency. Improper stencil cleaning method and frequency will result in incomplete stencil cleaning, continuous tin electrodeposition or insufficient tin in the stencil apertures will result in narrow space products.
Equipment accuracy
In high-density, small-space printing products, the printing accuracy and repeatability of printing will affect the stability of solder paste printing.
PCB Support
PCB support is an important adjustment content for solder paste printing. If the PCB lacks effective or improper support, thick or uneven solder paste should be used. The PCB supports should be arranged flat and evenly to ensure tightness between the stencil and the PCB.
Component Installation
The quality of component installation depends on several factors:
Choose the Right Components
Accurate placement and suitable installation pressure. Proper selection of components refers to the fact that components must be compatible with the requirements of the BOM. Correct placement means that the coordinates must be correctly mounted, and the accuracy of the mounting procedures must ensure the stability of the mounting and the correct mounting of the component on the pad.
Attention Must Be Paid to the Installation Angle
to ensure the correct orientation of the components. The proper mounting pressure refers to the pressed thickness of the component and must never be too small or too large. The placement pressure can be determined by setting the PCB thickness, component package thickness, the placement machine pressure of the nozzle and adjusting the placement machine’s Z-axis.
Reflow Soldering
Soldering quality of solder joints depends on the correct setting of the reflow profile. A good reflow soldering profile requires that all mounted components on the PCB must receive excellent soldering and that the solder joints should be both good looking and high quality.
If the temperature rises too fast, on the one hand, the components and the PCB will be heated so that the components are easily damaged and the PCB deforms.
On the other hand, the solvent in the solder paste volatilizes too quickly and the metal compound sputters out as tin balls. The peak temperature is usually set 30°C to 40°C higher than the melting point of the solder paste. If the temperature is too high and the reflow time is too long, it will damage the heat resistant components or the component plastic.
Conversely, incomplete melting of the solder paste results in a reliable solder joint. To improve soldering quality and prevent component oxidation, nitrogen reflow soldering can be applied.
The reflow profile is usually set according to:
Set according to the recommended temperature profile of the solder paste. The composition of the solder paste determines its activation temperature and melting point.
Depending on the thermal performance parameters of heat-resistant and valuable components, for some special components, the maximum soldering temperature must be considered.
Set according to the material, size, thickness and weight of the PCB substrate.
Set according to the structure of the reflow oven and the length of the temperature zone, and different reflow ovens should have different settings.
There are many factors that affect the quality of SMT welding:
Solderability of Components Included
PCB quality
PCB pad design
solder paste quality
PCB manufacturing quality
SMT Manufacturing Equipment Status
The technical parameters of each link of SMT and the skill operation of each engineer
Among these elements, the quality of components, PCB and solder paste, and PCB design are critical for reflow soldering quality assurance, because soldering defects caused by these elements are difficult or impossible to resolve with technical solutions.
Therefore, the first condition to improve excellent soldering quality is good control of material quality and excellent PCB pad design, which can reduce the impact.
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