As discussed in our earlier blogs, Solder Paste Printing is one of the key stages of the SMT process. In this stage, the Solder Paste is evenly applied to the PCB or the Printed Circuit Board using a stencil or foil or through jet printing. If the Solder paste isn’t properly applied, it could lead to major assembly defects.

For an effective solder paste printing process, certain parameters need to be kept in mind, let’s discuss the same.

1. Speed, Pressure & Angle of the Squeegee Blade: The speed, pressure and angle at which the Squeegee blade is placed are essential parameters to determine the ideal Solder Paste application. The speed, for example, determines the available time for the solder paste to roll into the apertures of the stencil and onto the pads of the PCB. Similarly, sufficient pressure across the entire squeegee blade length ensures a clean wipe of the stencil. Low pressure can cause smearing of the paste on the stencil whereas too much pressure can cause bleeding of the paste between the stencil and PCB. Usually, the squeegee blade is placed at an angle of 60 degrees, more than that can cause scooping of the holder paste and a lesser angle can cause a residue deposit of solder paste on the stencil after the squeegee has completed a print.
2. Stencil Separation Speed: After printing, the speed at which the stencil is separated from the PCB is the Stencil Separation Speed. It is determined by the size of the apertures within the stencil. Usually, a speed setting of up to 3mm per second is considered ideal. Anything faster than this can cause ‘dog ears’ i.e. formation of high edges around the deposits.
3. Stencil Cleaning: One must be mindful of cleaning the stencil regularly. Many automatic machines have the in-built command of cleaning the stencil after a fixed number of prints. It can also be cleaned using lint-free material applied with cleaning chemicals. If not cleaned regularly, it can cause bleeding on the underside of the stencil and block the apparatus.
4. Condition of the Stencil & Squeegee Blade: In addition to cleaning the stencil and the squeegee blade, one must also be mindful of the storage when not in use. The stencil and the squeegee blade must be carefully stored to avoid any damage as it can lead to undesired results. Prior to using either, they must be thoroughly checked and if any damage is noticed, they must be replaced to ensure a reliable and repeatable process.
5. PCB Support during Printing Process: It is important to ensure that the PCB is well supported during the printing process. Usually, supports are provided with printing machines which can be fixed at specific heights and have programmable positions to ensure a consistent process. If the PCB is not fully supported it can lead to printing defects such as a poor paste deposit and smudging.
6. Adequate Print Stroke: Print stroke is the distance the squeegee travels across the stencil and rolls back to generate the downward force that drives the paste into the apertures. The distance the squeegee travels is recommended to be a minimum of 20mm past the furthest aperture to allow enough space for the paste to roll on the return stroke.
7. 2D /3D Inspection of the Process: The process must be verified using automated inspection which is usually of two types – 2D inspection which checks the area of the paste deposit and 3D inspection which checks the volume of the paste deposit. Inspection is required to accurately check solder paste deposits.

The printing stage of the assembly process is vitally important. If any errors occur during this stage, it can affect the entire assembly process. To develop the optimum solder paste printing process it is essential to consider all aspects of the process detailed above.