There are so many processes involved in the surface mount assembly however, applying solder paste to the printed  circuit board (PCB) is a part among the many significant processes in surface mount assembly. This outcome is collectively accomplished by employing the usage of stencil or foil.

A stencil can be described as a sheet of material that is lightweight, commonly stainless steel. It has sequence of holes (apertures) carved inside which is appropriate for the printing of the PCB.

Putting the correct quantity of solder paste on top of the surface mount pads is the purpose of the solder paste  stencil. This is because the solder connection between the component terminal and PCB pad with respect to the mechanical strength and electrical connection have to be bearable.

Considerations

The basic design properties required of an effectual solder paste stencil are in this way:-

· Aperture design

· Stencil thickness

· PCB design specific

· Stencil material and mounting

· Stencil alignment

Stencil Thickness

This is relevant as it make sure that the discharge of solder paste from the apertures (holes) is reasonable as well as getting the accurate volume of solder paste required to attain the solder joint that you want.

Usually, the thickness of the stencil is between the range of 4 thou and 8 thou thick.

Perhaps the thickness of the stencil is not suitable for the size of the holes (apertures), in that case, it may give on to griping of the paste to the inner walls as a result of surface tension.

The following is a formula that can be used to select the appropriate stencil thickness

A formula below which can be useful in choosing the suitable stencil thickness:-

In addition, the discharge of solder paste out of the holes (apertures) is influenced by the specific size of solder paste choosen.

You can see below the specific sizes that are accessible:-

In existence is a general-principle stating "ideally a minimum of 5 solder particles should span the width of the smallest aperture."

Aperture Design

It is key to cease solder defects from making for example solder beads and bridging.

The designed of majority of apertures are a bit little compared to the pads (it is usually 2 thou reduction overall) in order to give a quality gasket seal between the PCB and the stencil. However, not many apertures require a particular design to put smaller paste onto the inner edge of the part to lower the chances of 'mid-chip' being formed.

An instance of an attainable solution called 'home-plate' apertures and a photograph of 'mid-chip' solder balls.

This plan of action is effectual for elements down to 0603 size, once 0402's or little are employed it can enlarge the possibilities of tombstoning.

For little size elements like these, suggestion is given to just have the  standard 2 reduction or leave 1:1.

In addition, the design of the PCB pads has a significance following the solder joints. Therefore, it suggested for reference to be made to IPC standard IPC-7351 once the definition of surface mount pad dimensions is made.

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