Solder Paste Stencil

The manufacturing industry focuses on the outcome of the process to earn more profit in lesser time. Hence in the electronics manufacturing industry, where PCBs play a vital role in the development of advance electronic products, PCBs are being manufactured at very high rapid pace. The majority of PCBs we see nowadays are multilayer.

The multilayer PCBs mainly composed of SMT components being assembled upon the board by automated setup of robotic arms which use the Pick and place files being generated by PCB layout designer. This pick and place file gives coordinates of each and every component on the PCB and a robot will accurately pick component from reel and place the component on desired location.

 

Similarly, the PCB layout design has a specific layer of PCB called "solder paste layer" or "cream" layer. There are top and bottom solder paste layers. Many PCB layout CAD software like Altium and EAGLE have this solder paste layer. This solder paste layer in the PCB layout gives information about the placement of solder paste on the PCB pads.

 

As we know that the SMT components do not have legs so these components needs solder paste to be applied on solder pads before the "reflow soldering" happens. The reflow soldering is the method of baking the PCB after application of solder paste and placement of components on PCB by securing them with glue or flux. The reflow oven will give heat treatment in predefined controlled manner called "Temperature Profile". This controlled temperature will melt the solder paste and make a strong connection between component lead and solder pad.

 

In this article we will know about the method of applying this solder paste and tools used in the process. The main tool used is called "Solder Paste Stencil".

 

What is Solder Paste Stencil..?

 

The solder paste stencil is the sheet of metal like stainless steel or nickel that has holes cut very accurately according to the solder paste layer of PCB layout design. This cutting is done by highly precise "laser cutting machine".  This method of laser cutting is costly and requires extra care to properly fix the stencil on the PCB and avoid any movement while applying solder paste. The stainless steel stencil is suitable for large production run where large number of PCBs are needed to be solder pasted. A single stencil of stainless steel can cover up-to thousand PCBs. However this stencil is costly for only few prototype PCB fabrication.

 

For few pieces of prototype PCB to be fabricated and solder paste screen printed it is recommended to use polyimide stencil which is lot cheaper than stainless steel and is easier to make these polyimide stencils.

 

 

 

 

You just need to print and cut the solder paste layer on this polyimide sheet using laser.

 

The stainless steel stencils are however gives high quality results, have trapezoidal opening, but manufacturing speed of stainless steel stencil is slow. The holes made by laser on the sheet of metal is kept a little smaller than the pad size on PCB so that it will not overspread and will not come out of PCB pad boundary.

 

 

Solder Paste:

 

The solder paste is SN63/Pb37 alloy very frequently used as solder for screen printing on PCB pads. This solder paste requires force being applied by the "applicator" or "squeegee" to evenly spread the solder paste on the pads through the aperture walls of stencil.

 

 

 

The amount of solder paste being applied on the pads is proportional to the viscosity of solder paste, aspect and area ratios of stencil.

 

The Squeegee:

 

The squeegee is the hard blade of metal use to uniformly spread the solder paste on the stencil to screen print the solder paste on PCB pads. The angle of squeegee against the surface of stencil, pressure or force applied on squeegee and direction of movement of squeegee against the stencil surface will determine how properly the solder paste is applied. The squeegee speed should be 25mm/sec and squeegee force should be 500gm/25mm of squeegee blade length and angle of squeegee should be 60O. The separation speed of stencil after solder paste is applied should be up-to 3mm/sec.

 

 

 

 

The Stencil Area and Aspect Ratios Formulae:

 

 

 

 

Types of Solder Paste Stencils:

 

The stencil can be made of paper, Mylar, polyimide and stainless steel. The thickness and size of aperture opening will determine the amount and volume of solder paste applied. The components like 0603 capacitor and resistor and 0.02” pitch SOICs will require thinner stencils while components 1206 and 0.05” capacitor and resistor and ICs will require thicker stencils. Typically the stencil thickness is 0.005” to 0.007”. The stainless steel stencils are robust and recommended for production run. The aperture opening is recommended to keep less in size than pad size. The aperture opening is about 10% less than the size of the pad. This is done while designing the PCB and the solder paste top and bottom layers in Gerber files are used to print the stencil image on stencil sheet of Mylar or stainless steel or polyimide.

 

Framed Stencil:

 

The stencil that comes already fitted inside the fix frame. The frame is a solid or hollow aluminum frame to hold the stencil. These stencils are used for high volume screen printing and is relatively costly. They are recommended for production run. The commonly available size of framed stencil is 20” x 20”.

 

Frameless Stencil:

 

These are also called universal stencil and they allow changes to be made to the stencil because stencil is not fixed in any frame. You do not have to completely replace the stencil unlike the framed stencil. Usually the frameless stencil is used for prototype purpose and not for production runs. The users order the frameless stencils usually already have frames that they will attach the stencil manually. They are less expensive than their framed stencil counterparts and requires less storage space.

 

Rayming PCB Stencils:

 

You can throw your inquiry for solder paste stencil at this email sales@raypcb.com.

You can check the Rayming PCB stencils price list and types at this link. Ray PCB Stencil Prices