The application of solder paste is one of the most crucial processes in the SMT assembly. This process requires careful attention and a high level of precision. This is because the main goal is to apply solder in the proper location and adequately. Therefore, it is important to carefully coat each pad using the appropriate amount of solder.
The most common method integrated is screen-printing the solder paste. Solder printing is the most delicate step in the printed circuit board assembly. Printed circuit boards have defects due to the use of poor methods and tools for applying solder paste. Solder paste containing lead are not RoHS compliant. It is advisable to apply lead-free solder paste on printed circuit boards.
Rayming PCB makes use of automatic solder paste printers and a squeegee blade to apply solder paste on a circuit board. Also, this PCB assembler ensures enough pressure is applied to completely cover the circuit board with paste by dragging a squeegee blade through the pcb stencil printer, which has been tailored according to the IPC-7525A. These blades are typically made of metal. Here is a list of the key elements that determines effective solder paste printing.
What to Consider During Solder Paste Printing
Squeegee Speed
The amount of time the solder paste has to “roll” in the stencil’s apertures as well as onto the PCB pads is determined by the speed at which the squeegee travels. The standard setting is 25 mm every second, however, this varies according to the aperture size within the solder paste and stencil used.
The Squeegee Pressure
To guarantee a thorough wipe of a stencil throughout a print cycle, it’s critical to apply enough pressure over the whole length of your squeegee blade. Inadequate pressure might lead to incomplete transfer into the PCB, inadequate deposition, and “smearing” of that paste onto the stencil. When there is too much pressure, this can cause paste “bleeding” between the PCB and stencil, “scooping” paste from bigger apertures, and too much wear from the squeegees and stencil. A normal setting for the pressure of the squeegee is 500 grams for every 25mm of the squeegee blade.
The Squeegee Angle
Those holders to which the squeegees are attached usually set their angle at 60 degrees. If there is an increase in angle, the solder paste deposited will be minimal since the paste will be “scooped” out of the stencil apertures. Any reduction in the angle may result in solder paste residue remaining within the stencil once the squeegee concludes printing.
Speed of Stencil Separation
The speed of stencil separation describes the rate at which the circuit board gets separated from its stencil. The apertures’ size within a pcb stencil printer will determine the speed setting, which can range from zero to three millimeters per second. If this happens too quickly, the solder paste won’t completely release from these apertures, and “dog-ears”—high edges surrounding the deposits—will appear.
Cleaning the Stencil
Cleaning the stencil needs frequently is recommended while it is being used; the PCB assembler can perform this manually or mechanically. A good number of automatic printing devices feature a system you can configure to clean a stencil using lint-free material and a cleaning agent such as IPA after a using a certain amount of prints. The system executes two main tasks which is it cleans the stencil’s underside to prevent any form of smudging, and again it makes use of suction to clean off the apertures to prevent any form of obstructions.
Squeegee and stencil condition
It is important to carefully maintain squeegees and stencils since mechanical damage to any of these tools can lead to unintended effects. You need to thoroughly inspect both of them before they are used and as well ensure they are cleaned, ideally with an automatic cleaning system which can get rid of any residue of the solder paste. In case of any damage to squeegees and stencils, replacement must be done as soon as possible. This helps to guarantee a dependable and reproducible procedure.
PCB Support
It’s crucial to have PCB support so that the circuit board stays flat on the stencil during printing. The poor paste deposits as well as smearing can occur during printing when the PCB isn’t adequately supported..
To maintain a uniform process, circuit board supports are typically included with the printing machines. These supports are fixed in height and feature programmable settings. Additionally, there are available adjustable PCB supports with a variety of designs that conform to the circuit board and are very important for two sided assemblies.
The Print Stroke
The squeegee should be at least 20 mm beyond the farthest aperture when traveling this distance through the stencil. As the bead of the solder paste rolls on its return stroke, creating a downward pull that forces the paste through the apertures, traveling past the farthest aperture is crucial for allowing adequate room for the rolling of the paste.
Type, Handling, and Storage
Solder paste is a combination of solder and flux. The components are held in place by the flux, which serves as temporary adhesive, until the soldering process melts that solder and creates the mechanical/electrical connection.
Because solder paste constitutes a ‘Thixotropic’ substance, energy is needed to alter its flow and viscosity uniformly through these stencil apertures. This energy comes from the velocity of the print head. The solder paste’s ‘Rheology’ is a term that describes how this solder paste creates a block without the application of energy but changes into a material which is more fluid whenever there is application of energy.
The apertures’ size within a stencil should determine the appropriate solder paste type to use. The chosen solder paste’s particle size has an impact on the release of the stencil’s apertures.
According to the “5 ball rule,” at least 5 solder particles should be able to span the smallest aperture’s width. You need to keep the lead-free and tin-lead solder paste at room temperature for at least eight hours before the usage in order to retain quality. To extend the solder paste’s shelf life, you need to keep it chilled.
For uniform distribution of every separated material within the paste, the solder paste needs to be mixed before its use. You can mix the solder paste either manually or mechanically with the help of the paste conditioner machine. Ensure you mix for a period of three to five minutes.
Solder paste should generally be thrown away after being used for more than eight hours. However, you can keep solder paste at room temperature after being used for a maximum of four hours. Such solder paste can be kept in a sealed container before reuse up to a day.
Visualization (2D and 3D)
Automatic inspection may be utilized to precisely check the deposits of the solder paste to validate the procedure. There are also two kinds of inspection for solder paste. These are 2D inspection, that examines the paste deposit’s area, as well as 3D inspection, that examines the paste deposit’s volume.
What Varieties of PCBs may be Printed?
Irrespective of the type of PCB, whether rigid-flex, IMS, rigid, or flexible, if the strength of the circuit board can’t hold the PCB perfectly and as necessary on the surface mount lines, the circuit board assembly manufacturer would inquire if the carrier (Durostone made) or SMT carrier can be customized.
It is based on the quantities used in PCB assembly.
To make sure the printed circuit board is flat on the stencil throughout printing, this is a crucial factor. Printing flaws such as poor paste deposits as well as smearing can result from a PCB that doesn’t receive adequate support, regardless of whether it is rigid, IMS, rigid-flex, or flexible. To maintain a consistent process, PCB supports are typically included with printing machines. These supports are fixed in height while offering programmable settings. For double-sided assembly, flexible PCB are also available and helpful.
Solder Paste Inspection with Printing
Solder paste inspection is necessary after applying solder paste on a board. Errors can occur during the application of solder paste on a circuit board. The rule of thumb here is to fix a defect on time before it becomes more expensive. Early detection of errors in solder printing can help to prevent further errors.
A helpful rule to keep in mind is that a problem discovered after reflow would cost ten times as much to rework as one discovered before reflow, and a fault discovered after test would cost an additional 10 times as much.
It is acknowledged that there is more possibility for flaws or errors to occur during the process of solder paste printing. Also, the complexity of the printing process has increased with the use of small components and this has resulted in the use of lead-free form solder paste. According to experts, tin lead solder pastes spread and offer more wettability than lead-free form of solder pastes.
Generally, a lead-free technique necessitates a more precise printing procedure. The manufacturer has been compelled by this to execute some sort of post-print examination. Automatic inspection of the solder paste could be used to precisely check the deposits of the solder paste in order to validate the procedure. At RayMing PCB, we can identify a number of solder paste printing defect such as missing solder paste, the paste offset, spreading, bridging, and lines with insufficient or excessive deposits.
Printing using Low Temperature Lead-Free Solder Paste
To get a consistent, high-quality print while printing lead-free low temperature solder paste, it’s crucial to employ the right machine parameters. As previously mentioned, the standard print speed is 25mm for each second, and the standard squeegee pressure is 500 grams for 25mm of the squeegee blade.
If not configured properly, the settings enabling printing lead-free low temperature solder paste could be extremely different and result in smudgy prints. Printing at speeds of 50 to 100 mm every second should be substantially faster than with traditional solder pastes. This setting should be closer to a pressure of 300 grams for every 25mm of the squeegee blade. The squeegee pressure has to be lower than typical solder pastes and it is advisable to utilize no additional pressure than necessary to get the clean stencil during printing.
The quantity of the solder paste applied to a stencil should also be considered. Since the stencil it is lighter weight and has less tin than lead-free low temperature solder paste, it can adhere to squeegee blades while it is in use. In order to make the paste remain within the stencil inside the continuous bead, ensure you apply more paste compared to ordinary solder.
An increase in productivity and efficiency
It’s surprising that several surface mount processes within our electronics industry, especially those in subcontract manufacturing, operate at as little as 20% in terms of efficiency. This percentage is because of the various factors. However, it basically indicates that just 20% of this capital invested is being used. As regards financing, there is more ownership cost, and also a lower rate of return on investments. There may be a longer lead time for the customer’s products and as a result, the company won’t stay competitive within the market.
The firm will be impacted by a number of consequences when there is a low level of production efficiencies. This includes more batch sizes, additional components in stock, extra assemblies in progress, as well as slower response times to modification requests by customers.
With all the factors mentioned above, there is a reason to increase efficiency while still ensuring quality.
Conclusion
Solder paste printing is a crucial step in the PCB assembly process. When there are defects at this point, there’ll be “knock on” consequences all through the process. Therefore, all components of the process described above must be looked into so as to achieve an ideal solder paste printing procedure. You can determine the quality of solder paste by carrying out a coalescence test . This test can also help to determine how the working atmosphere (relative humidity and ambient temperature) would have an impact on performance.
