Solder Paste Printing Bad Reasons and Countermeasures

The Foundation of SMT Welding - Solder Paste Printing
How to deal with solder paste problems such as collapse, short circuit, missing or offset?
Suzhou Nofel provides you with detailed answers.
According to data analysis, approximately 60% -70% of soldering defects in the SMT assembly process of electronic products can be attributed to poor solder paste printing quality. Therefore, the quality of solder paste printing is directly related to the yield level of SMT assembly. The key to achieving zero defect production lies in strictly controlling the quality of solder paste printing to avoid welding defects caused by poor printing.
In SMT solder paste printing operations, common defects mainly include the following:
1. Solder paste collapse problem
2. Uneven solder paste printing area
3. Analysis and treatment methods for solder paste short circuit phenomenon
1. Inaccurate solder paste position
2. Center offset of solder paste
3. Solder paste printing deviation
4. Solder paste positioning error
5. Solder paste deviates from the center
5. Quality control of solder paste printing
6. Solder paste edge defects
7. Solder paste removal or scratch treatment
8. Solder paste contamination, surface blurring
9. The amount of solder paste used exceeds the standard value
1、 Solder paste collapse phenomenon
The printed solder paste failed to maintain its stable shape, resulting in edge collapse and gradually spreading towards the periphery of the solder pads, forming conduction between adjacent solder pads. If not dealt with in a timely manner, this situation will inevitably cause a welding short circuit during the reflow soldering process.
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The reason analysis has been modified as follows:
**Deep Analysis of Reasons**
If the pressure applied by the scraper is too high, the solder paste will be subjected to excessive compression and may seep into the gaps between the steel mesh and the PCB board. In severe cases, the solder paste between adjacent pads may adhere, leading to collapse issues.
If the viscosity of the solder paste is too low, viscosity becomes a key indicator for maintaining the shape of the solder paste. If the viscosity is insufficient, the edges of the solder paste are prone to relaxation and collapse during the printing process, especially when dealing with fine pitch components, which may cause solder paste short circuit problems.
If the metal content is too high, when the solder paste is left on the steel mesh for a long time or recycled solder paste is used, due to the volatilization of the diluent component, the metal content remains unchanged, which may lead to a decrease in the viscosity of the solder paste and cause collapse.
4. Solder particles are smaller, especially solder paste with smaller particle sizes, which has better flowability under the steel mesh. However, during the printing process, the shape retention of solder paste is relatively poor. In addition, due to the easy diffusion of metal particles under the steel mesh, it may lead to the phenomenon of solder paste short circuit during the printing process.
5. Moisture absorption characteristics of solder paste: When the air humidity is high or the solder paste is exposed to air for too long, it may cause the solder paste to absorb moisture from the air, leading to dilution, viscosity decrease, and inability to maintain its original shape, resulting in collapse.
When the ambient temperature rises, the viscosity of the flux in the solder paste will correspondingly decrease, which may lead to the collapse of the printed solder paste. In addition, if the duration is too long, the diluent components in the solder paste will gradually evaporate, causing the viscosity to increase and increasing the difficulty of printing.
2、 Uneven solder paste application area
The solder pad coverage area refers to the surface portion of the solder pad that needs to be filled with solder paste. Ideally, the area of this area should match the opening area of the steel mesh. However, in practical operation, the actual coverage area of solder paste on the solder pad may be less than or exceed the opening area of the steel mesh. If the solder paste coverage area is insufficient for the solder pad, it may cause a shortage of solder; On the contrary, if the coverage area is too large, it may cause short circuits or excessive tin.
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Root cause analysis
If the solder paste is not operated properly during the demolding process of the steel mesh, or if the printing parameters are set improperly such as the demolding speed, it may cause irregular edges of the solder paste during demolding of the steel mesh, and even the solder paste may be embedded in the steel mesh holes. This phenomenon will cause solder paste to overflow from the edge of the solder pad or partially expose the solder pad, leading to problems such as short circuits and insufficient solder.
2. In the printing process, if the amount of solder paste on the steel mesh is found to be insufficient, it needs to be replenished regularly. If the solder paste is consumed too quickly, the amount of solder paste filling in the hole will decrease, and it will not be able to fully cover the solder pad, thereby causing the problem of "insufficient solder" due to poor soldering.
3. Solder paste drying problem: When the solder paste is left on the steel mesh for too long, the diluent evaporates and the solder paste will dry. In this case, the thixotropy of the solder paste will deteriorate, making it unable to effectively fill the mesh and may even adhere to the pore wall. In addition, the amount of solder paste deposited on the solder pads may be insufficient or completely missing.
If the bottom of the steel mesh is contaminated and not cleaned in a timely or thorough manner, the solder paste will adhere to the bottom of the steel mesh and gradually form lumps. This phenomenon will increase the gap between the steel mesh and the PCB solder pads. Usually, this leads to an increase in the volume or coverage of solder paste on the solder ribs. However, it is also possible to have a shortage of tin, as these contaminants may remove the solder paste on the solder pads.
5. Due to excessive collapse, moisture absorption, or other factors, the solder paste may collapse at the edge of the solder pad and overflow outside the solder pad.
6. The printing speed is too fast, causing the solder paste to slide on the surface of the steel mesh and unable to effectively fill the holes in the steel mesh. Therefore, the insufficient filling of solder paste in the hole prevents the solder paste deposited on the pad from fully covering the pad.
7. Excessive pressure causes solder paste to seep out from the bottom of the steel mesh and overflow onto the surface of the solder pad.
3、 Short circuit phenomenon and treatment methods of solder paste
The solder paste bridging formed between adjacent pads is called wet bridging. During the reflow soldering process, wet bridging may sometimes automatically detach due to the surface tension caused by the melting of the solder. If the bridge cannot detach, welding defects such as short circuits may occur.
Original text: Cause analysis
Revised text: In depth analysis of the reasons behind it
1. The collapse phenomenon of solder paste may be caused by the characteristics of the solder paste itself, such as metal content, size of solder ball particles, or control factors of the solder paste, such as moisture absorption. In addition, printing parameters such as printing pressure and demolding speed may also be influencing factors. If the solder paste deposited on the PCB pad cannot maintain its original shape, it will collapse and form a bridge connection.
2. If there are stains or foreign objects attached to the bottom of the steel mesh, it will widen the gap between the PCB and the bottom of the steel mesh, causing solder paste to overflow under the steel mesh during the printing process, thereby causing bridging phenomenon.
3. Excessive pressure causes solder paste to overflow from the bottom of the steel mesh, covering the solder pads and causing bridging phenomenon.
If the gap is set too wide, once there is a large gap between the steel mesh and the solder pad, it may cause solder paste to overflow from the solder pad, resulting in bridging phenomenon.
5. The steel mesh is damaged or broken, causing the partitions between adjacent openings to break or deform, thus forming a bridge connection during the solder paste printing process.
6. The thickness of the solder mask is uneven, and some areas may exceed the height of the solder pad. In the process of printing solder paste for fine pitch components, especially for CSP (chip level packaging) or connector components with a size below 0201 or a spacing below 0.4mm, if there is a lack of solder mask isolation between adjacent pads, the solder paste may overflow the edge of the pad, resulting in bridging phenomenon.
7. Inaccurate positioning of steel mesh or PCB, incomplete alignment of steel mesh openings with solder pads, solder paste printing beyond the range of solder pads, resulting in reduced gaps between solder paste and adjacent solder pads. During the process of mounting components, the compression of solder paste by the components may cause the solder paste to connect with adjacent pads, and after reflow soldering, a short circuit may form.
When performing multiple printing operations, especially to increase the amount of solder on the solder pads or ensure sufficient solder paste filling for small hole components, sometimes two printing processes are used. However, this method may lead to excessive squeezing of solder paste, which can cause collapse and short circuit problems.
9. PCB contamination issue: When dirt or other impurities appear on the PCB, the mechanism is similar to the situation of steel mesh bottom contamination. The direct reason is that it increases the gap between the steel mesh and the solder pad, causing the solder paste to overflow from the solder pad and trigger a short circuit.
4、 Solder paste offset issue
In the process of printing solder paste, if there is incomplete alignment with the actual pad position, it may cause bridging problems and also lead to solder paste overflowing onto the solder mask, forming solder balls.
Root cause analysis
1. Insufficient clamping or support of the circuit board, inappropriate parameter settings, or deviations in the thickness and size of the PCB can all cause displacement of the circuit board during the printing process, leading to misalignment between the steel mesh and PCB, and ultimately resulting in positional deviation after solder paste printing is completed.
2. Deviation in PCB material dimensions or PCB deformation after a reflow soldering process may cause matching issues between solder pads and steel mesh openings. In this case, the mesh and solder pads cannot achieve precise alignment, resulting in a deviation in the printing position.
3. There may be errors in the calibration program, which may be due to program configuration errors, inaccurate device parameters or PCB size settings, leading to printing position offset.
4. The circuit board is deformed, and there is a deviation in the relative position between the steel mesh holes and the solder pads, which leads to positional displacement problems.
5、 Quality control of solder paste printing
When the solder paste on the solder pad is not fully deposited, this phenomenon is called leakage, manifested as the solder paste coverage area being less than 80% of the opening area. This situation can lead to insufficient soldering of solder joints, and even complete wetting of solder pads and component terminals with solder paste, which is unacceptable.
Original text content: Reason analysis
Modified text content:
Cause analysis
1. The scraper speed should not be too fast, otherwise it may cause the solder paste to slip and affect the filling effect, especially when dealing with micro holes.
2. The separation speed should not be too fast: After the solder paste printing is completed, if the separation speed is too fast, it may cause the solder paste on some pads to be excessively scraped off, resulting in missed printing defects.
3. In the printing stage, if the amount of solder paste on the steel mesh is insufficient, it will lead to insufficient filling of solder paste in the holes during the printing process, thereby causing the problem of insufficient solder amount.
When the steel mesh is damaged, deformed, or otherwise damaged, it may cause blockage of the mesh holes, leading to insufficient solder paste leakage.
If the size distribution of solder particles is too broad, it will affect the surface mount (SMT) process quality of micro or high-density spacing components. At this point, the amount of solder paste deposited on the PCB pads often fails to meet the standard requirements. Usually, the size of metal particles should follow the "5-ball principle" to ensure process stability.
6、 Uneven solder paste coating
The deposition of solder paste on the substrate should ensure clear contours and regular shapes, avoiding problems such as dog ear shape or unclear contours. Poor printing boundaries are unacceptable as they may cause uneven solder joints. Even if the total amount of solder paste is correct, if the distribution is uneven, solder paste bridging may still occur during reflow soldering. The appearance of dog ears, protrusions, uneven edges, and indentations are all examples of poor printing boundaries.

Original text: Cause analysis
Revised: Factor Analysis
1. The bottom of the steel mesh is contaminated/not thoroughly cleaned. If there is contamination or inadequate cleaning at the bottom of the steel mesh, the solder paste on the hole wall may be carried away during the process of solder paste printing or steel mesh demolding, leading to irregular edges, collapse, or missing edges.
2. If the separation distance is too narrow, it may cause the solder paste to not fully detach from the steel mesh wall, resulting in excessive stretching of the edge solder paste.
3. If the separation speed is too fast, it will not be conducive to the effective separation of the steel mesh wall and solder paste, which may lead to poor edge forming.
4. Low printing efficiency leads to excessive compression of solder paste in the solder pad holes, resulting in excessive solder paste usage. When the scraper is removed from the steel mesh, the pressure of the solder paste is suddenly released, causing the solder paste to rebound, which in turn causes the steel mesh to separate from the solder paste, resulting in irregular formation of the solder paste.
The selection of steel mesh thickness and opening size is crucial, and its area ratio needs to be fully considered. Generally speaking, the area ratio should not be less than 0.66. If the area ratio is too low, it will be detrimental to the filling and demolding effect of solder paste.
6. The distortion and deformation of the steel mesh affect the forming quality of the solder paste at the edge, resulting in poor demolding effect.
If the solder paste is left on the steel mesh for too long, it may cause the diluent to evaporate excessively, leading to drying. After the solder paste dries, its viscosity increases, which seriously affects the quality of demolding.
8. The printing gap is too large, causing the solder paste to spread under the steel mesh and form irregular solder paste shapes.
7、 Solder paste removal or wiping treatment
In the printing process, the so-called scraping phenomenon refers to the removal of solder paste from the central part of the mesh of the printing screen or the formation of voids. This phenomenon is usually caused by the moderate hardness of the rubber scraper, which deforms under pressure and cuts into the hole, or by the large opening size of the steel mesh, causing the scraper to scrape off the solder paste inside the hole during the printing process. This phenomenon can lead to a decrease in the amount of solder in the solder joint, thereby affecting its strength and reliability.
Root cause analysis
The blade of the scraper is made of soft polyurethane material, while the hardness of the rubber scraper is lower. During the printing process, if the scraper deforms due to insufficient hardness, it may cut into the steel mesh, causing the printed solder paste to be excavated and taken away, resulting in scratch problems.
2. Analysis of excessive pressure on the scraper: For rubber scrapers, the reasons are as mentioned earlier. For some cases with larger opening sizes, even steel scrapers may deform due to excessive pressure, leading to adverse effects.
3. The steel mesh is damaged, and some of the separation areas between the steel mesh holes are broken, resulting in an increase in the aperture and causing the scraper to cut into the holes during the printing process.
If the blade of the scraper is damaged, the damaged part may be embedded in the solder pad hole during the printing process, thereby carrying away some of the solder paste.
8、 The solder paste is contaminated, blurry, has tailing phenomenon, or the solder paste in the pad area is missing.
·Fuzzy phenomenon refers to the contamination of tin dots on the surface of the board during the printing process or subsequent steps.
·This type of problem usually occurs when solder paste printing is mistakenly located in non-standard areas, solder pad gaps, and other positions.
Due to the possibility of bridging or tin balls, it is not acceptable.
During the printing process, if the solder paste on the solder pads is partially removed due to blockage of the steel mesh holes or uneven solder paste printing, if the printing machine forcefully drives when separating the board, it may cause further detachment of the solder paste. This situation can cause insufficient deposition of solder paste on the solder pad, thereby reducing the mechanical strength of the solder joint and affecting the overall reliability of the circuit.
Root cause analysis
During the gap printing process, if the gap between the steel mesh and the PCB board is too narrow, it may cause adhesion between the steel mesh and solder paste. This may result in excessive removal of solder paste when outputting the circuit board.
2. Unstable clamping of circuit boards, inaccurate machine parameter settings, board thickness settings lower than actual thickness, loose fixtures, and other factors may all cause displacement of PCBs during the printing process, leading to poor printing.
3. Insufficient pressure applied by the scraper caused the solder paste to not fully fill the holes on the steel mesh, resulting in incomplete solder paste coverage in some areas.
4. After completing the solder paste printing, if it is necessary to manually handle the PCB board due to production material or SMT machine failure, be sure to operate with caution. Avoid touching the printed solder paste with hands or other objects to prevent partial detachment of the solder paste, which may result in insufficient solder volume at the final solder joint.
9、 The amount of solder paste used exceeds the standard regulations
By utilizing advanced 3D solder paste inspection technology to measure the volume of circuit boards, the actual amount of solder paste can be accurately calculated, effectively avoiding welding defects such as short circuits caused by insufficient or excessive solder in solder joints.
Root cause analysis
If the thickness of the steel mesh does not match the size of the opening or if the steel mesh deforms, it will cause an increase in the amount of solder paste deposition, resulting in excessive solder paste usage and making the solder joints appear full. However, for micro components with fine spacing or dense arrangement, short-circuit problems are difficult to avoid in this case.
2. Improper gap setting: During the printing process, the ideal gap between the steel mesh and PCB should be 0 to achieve a tight fit. If the gap is too large, it may cause the solder paste to expand in volume, and in severe cases, even overflow from the solder pad, leading to welding short circuit problems.
If the height of the solder mask is higher than the height of the solder pad, that is, it is not level with the surface of the solder pad, especially when the solder mask is higher than the upper surface of the solder pad, for components with thin spacing, there may be a lack of effective isolation between the solder pads due to the solder mask. This situation is equivalent to forming a gap between the solder pad and the steel mesh, which may cause the solder paste to overflow from the edge of the solder pad, significantly increasing the risk of short circuit events.
4. Insufficient printing support. During the printing process, the pressure applied by the scraper may cause deformation in the middle part of the PCB board, leading to excessive deposition of solder paste.
5. The distortion and deformation of the circuit board will cause the contact surface between the steel mesh and the PCB to be unable to maintain sufficient tightness, resulting in excessive application of solder paste.
6. The scraper runs too fast and the printing speed is also very fast, resulting in insufficient rolling of the solder paste and sliding on the steel mesh. This situation makes it difficult for solder paste to effectively fill the steel mesh holes, resulting in insufficient volume of solder paste inside the holes.