Solder paste is a critical material used in surface mount technology (SMT) to connect surface-mounted components to PCBs through reflow soldering.
Solder paste alloy contains leaded solder with Sn63/Pb37 or lead-free solder with flux to remove and prevent oxidation. Solder paste is very important, especially in PCB assembly, in reflow soldering, solder paste ensures the reliability of electrical connections and adjusts component alignment automatically.
In this topic, we will discuss how to apply solder paste in PCB assembly, how to enhance the quality of soldered components, and common defects that may occur in solder paste application, as well as how to avoid them.
Solder Paste Application Process
Preparation Work
- Paste Selection: Choose the appropriate solder paste (e.g., lead-free or lead solder paste) according to the PCB design and component type and ensure that it has a shelf life.
- Stencil preparation: Check the model, tension and opening of the stencil to ensure that the stencil is equal to the PCB pad without clogging or deformation.
- Equipment debugging: Adjust the printer parameters, i.e., speed of printing, pressure, demoulding speed, angle of scraper, etc., such that the parameters are compatible with the process requirement.
Paste Handling
- Rewarming: Take solder paste out of the chilled storage state (4-10℃) and place it in an airtight container to heat for 4-12 hours to reach room temperature (23±3℃).
- Stirring: Blend the solder paste with an automatic mixer or by hand stirring for 1-3 minutes to ensure that the solder powder and the flux contained in the solder paste are finely mixed in a way that avoids precipitation. After stirring, the container must be capped tight to prevent volatilizing of the flux.
- Note: The unopened solder paste can be stored at room temperature for 7 days after being warmed up. If it is older than 7 days, it must be scrapped. After the can is opened, it must be used within 24 hours and should not be refrigerated. In the event it has not been used for more than 20 minutes, the solder paste must be returned to the original container and mixed for 2 minutes prior to reuse.
Printing Operation
- Install Stencil: Properly install the stencil onto the printer. You can utilize the Mark point and position it to ensure the stencil is also properly aligned with the PCB.
- Add Paste: Place an appropriate amount of solder paste into the stencil, and spread the solder paste on the stencil uniformly with a scraper covering all the openings but not exceeding the range of the scraper. The width is about 20-30mm and height is about 10-20mm.
- Printing & Measuring: Switch on the printer, and the scraper prints solder paste with the stencil openings onto the PCB pads. The rolling diameter of the solder paste on the stencil needs to be measured during the print every hour and kept between 20-40mm. If less than 20mm, solder paste needs to be refilled. If it is greater than 40mm, then it has to be recycled properly. There should be adequate check so that volume of solder paste is balanced and there is no bridging or tin short.
|
Component Type |
Stencil Thickness |
Aperture Ratio |
|---|---|---|
|
0201/CSP (0.3mm) |
0.1mm |
1 : 0.9 |
|
QFP (0.5mm pitch) |
0.12mm |
1 : 0.95 |
|
BGA (0.8mm pitch) |
0.15mm |
1 : 1 |
Quality Control
- Visual inspection: Check whether the shape, position and height of solder paste printing meet the requirements, and whether there are any problems such as offset, collapse or bridging.
- Check paste volume (±15% of target)
- Verify height uniformity (CV ≤10%)
- Monitor coverage:
- 100% pad coverage for critical components
- ≥95% coverage for general SMDs
- ≤2 solder balls per 10cm²
Post-Printing Management
- SMT: Complete SMT within 2 hours of solder paste printing (maximum 4 hours for no-clean solder paste), and perform component SMT as soon as possible to avoid long exposure of solder paste to air oxidation.
- Reflow soldering: Put the PCB after SMT into the reflow oven and solder according to the predetermined temperature curve, using RSS or RTS curve, and determine the peak temperature according to the melting point of the alloy (e.g., SAC305 is 230-250℃), so that the solder paste melts to achieve a sound solder joint.
- Stencil Maintenance: Auto-clean 30min production + full wash 12hrs. Check tension after cleaning (≥30N center, ≥32N average)
Notes:
- Wear protective gloves when working to avoid skin contact with solder paste.
- The stencil must be cleaned and checked regularly to ensure that it continues to function steadily.
- Storage, reheat, agitation and usage of solder paste must strictly follow the first-in-first-out (FIFO) principle to ensure product quality.
- Different model and batch solder pastes must be stored separately to avoid mixing up.
- In the process of printing, care should be taken over the ambient temperature and humidity control (temperature 22-28℃, relative humidity 30-60%) to avoid environmental effects from affecting the solder paste performance.
Solder Paste Application Methods
There are a lot of techniques to apply solder paste on bare PCBs depending on production needs, like stencil printing, syringe dispensing, and jet printing. The most common method in production is stencil printing.
Stencil Printing
It’s the primary method for high-volume PCB assembly, and it’s used by most PCB manufacturers. To apply solder paste using a stencil, there are some guidelines that should be taken into consideration, like alignment of the stencil; it should be aligned on the PCB using fiducial marks. Misalignment of the stencil leads to shifting of the paste, leading to bridges when reflowing the solder. Fiducial marks can be placed by PCB designer or can be added by the manufacturer at an additional cost; they work as a coordinator for the board for stencil placement, pick and place machine, and for 3D inspection machines. Laser-cut stainless steel stencils are commonly used due to their precision and suitability for fine-pitch components.
Stencil thickness is another key factor. Standard stencils range between 4–6 mils. Thinner stencils are used for fine-pitch components, while thicker stencils are ideal for large pad applications.
Aspect ratio between aperture width and stencil thickness should be more than 1.5 and for fine pitches should be more than 0.66 to ensure proper solder paste release, Also squeegee angle and speed must be proper determined, too fast speed causes incomplete filling, and too slow speed causes lead smearing.
Solder Paste Dispensing
This method is used in low-volume production, proofing of prototypes, repairing or making reworks, and filling insufficient solder after stencil printing.
Dispensing can be done manually using a syringe or automatically with a dispenser machine. However, it is generally slower and less consistent than stencil printing, especially for fine-pitch components.
Jet Printing
It’s a jetting nozzle that sprays solder paste droplets on the PCB without any contact. Jet printing offers faster setup and flexibility for small runs or prototypes, though stencil printing remains faster and more efficient for mass production.
It can be classified as the best method for solder paste application, but it requires more cost because the machine and solder paste viscosity require precise control.
Critical Parameters for Optimal Solder Paste Application
To achieve high quality and reliable solder paste application, there are some key parameters that affect the quality and reliability, and print quality, like environment, solder paste properties, and accurate stencil design. We will talk about these parameters in detail.
|
Parameter |
Standard |
Tolerance |
|---|---|---|
|
Paste Viscosity |
180-220 Pa·s (Type 4) |
±20 Pa·s |
|
Stencil Tension |
32N (used) /45N (new) |
±2N |
|
Environment |
25±3°C, 40-60% RH |
±1°C/±5% RH |
Environmental Controls
Temperature, humidity, and stencil cleaning must be controlled properly; temperature has a direct relationship with viscosity. If the temperature is very high, solder paste will dry out, and if temperature is very low, the paste will become too viscous and its function becomes harder to control. The same for humidity; also, dust can clog stencil apertures, so the stencil must be regularly cleaned every 10 prints as a rule of thumb to prevent unexpected defects.
Solder Paste Properties
Solder paste contains paste, solder alloy, and flux as mentioned before; these properties should be adjusted according to the application's needs.
Military purposes require withstanding solder with high temperature; on the other hand, medical devices should be lead-free.
Paste must have moderate viscosity, not too low to be hard to control, or too high, causing poor stencil release. Also, the flux type must be clean and reliable to remove oxidation from the soldered area.
Stencil Design
Stencil Design key parameters that affect solder paste application are many, like aperture size and shape, stencil thickness, and alignment.
The aspect ratio between the aperture width and the stencil thickness should be reasonable to ensure proper release; also, the aperture should be tapered to improve aperture release also Misalignment of the stencil due to missing fiducials causes solder bridging or insufficient coverage.
Common Defects and Solutions in Solder Paste Application
After optimizing parameters, most of the expected defects should be avoided; however, there are some defects that may occur, we will list some of them and their solution.
|
Defect |
Causes |
Solutions |
|---|---|---|
|
Solder Paste Slumping (Paste spreads too much) |
Low viscosity, excessive stencil thickness, slow printing speed |
Use higher-viscosity paste, reduce stencil thickness, optimize print speed |
|
Insufficient Solder Paste (Weak deposits) |
Clogged stencil, low squeegee pressure, poor PCB support |
Clean stencil, increase squeegee pressure, ensure proper support |
|
Bridging (Solder shorts between pads) |
Too much paste, misaligned stencil, large apertures |
Adjust stencil design, improve alignment, reduce paste volume |
|
Solder Balling (Tiny solder balls near joints) |
Moisture in paste, oxidation, improper reflow |
Store paste properly, use nitrogen reflow, optimize profile |
|
Poor Stencil Release (Paste sticks to stencil) |
Dirty stencil, rough aperture walls, low-quality paste |
Clean stencil, use laser-cut/nano-coated stencils, choose better paste |
|
Paste Drying Before Reflow |
Long exposure time, high temperature, low humidity |
Reduce time between print & reflow, control environment |
|
Tombstoning (Component lifts on one side) |
Uneven paste deposition, imbalanced reflow heat |
Ensure symmetric pad design, optimize reflow profile |
|
Cold Solder Joints (Dull, weak connections) |
Low reflow temperature, old/expired paste |
Verify reflow profile, use fresh solder paste |
Conclusion
Thus, solder paste application is one of the critical steps for any PCB manufacturing company. It determines the PCB’s quality, performance and reliability. The right method to consider the parameters like paste properties, environment and stencil designs, every step helps for successful electronic products.
At FC PCBA, our advance solder paste application can help you whether you are prototyping or scaling up for mass production. Contact us today and get a free quote from our specialist.
Frequently Asked Questions
What inspection methods ensure good solder paste application?
Simply, you can visually check the quality of solder paste application; however, in high-volume production, you can inspect it using automatic optical inspection (AOI).
What is the best method for applying solder paste in PCB production?
Stencil printing is the best and most efficient method for PCB production because of its speed and precision.
How to keep solder paste viscosity in the accepted region during printing?
Maintain solder paste storage at 20–25°C and relative humidity around 40–60% to preserve viscosity and prevent degradation.
How to avoid bridges during soldering between tiny footprints like QFN?
Try to reduce stencil thickness, use tapered apertures to get a cleaner release, and check alignment and amount of solder paste before reflowing.
