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Optimizing Stencil Design for Electronic Component Assembly

Stencil Design for Electronic Component Assembly

The quality of solder paste printing is a critical step in surface mount electronic component assembly. In fact, studies have shown that the first time yield (FTY) of surface mount electronic/component soldering is directly related to the quality of stencil design. This is especially true for small pitch components where the size of the apertures and their position on the PCB play an important role in preventing soldering defects such as bridging, and solder bead formation.

In addition, the size of the apertures can affect how well the solder paste will transfer to the surface of the PCB during reflow. Aperture shape plays an important role as well. There are several different aperture shapes available and each has a different effect on the transfer of the paste to the PCB. The most common shape is the’squircle’ as it has the greatest transfer efficiency but there are many others that are also effective.

Stencil thickness is another key factor in reducing the possibility of defects. The optimum thickness is dependent on the component sizes, pitch and placement as well as the manufacturing process used. A thicker stencil will reduce the possibility of ‘tombstoning’ where the solder paste is deposited on top of a component – this can lead to a short between the component and the circuit board which can cause damage. On the other hand, a stencil with too thin a thickness can result in too little solder paste being printed on the circuit board which can lead to ‘open soldering’.

Optimizing Stencil Design for Electronic Component Assembly

How the stencil is manufactured can also have an impact on its ability to release paste from the apertures. Early stencils were chemically etched which gave good results for larger apertures but as component sizes reduced laser-cut stencils became the norm and offered much finer, trapezoidal aperture walls that facilitated better paste release. Mechanical polishing of the stencil is also an option which can improve the ability of the apertures to release paste with a higher level of precision.

There are some situations where the PCB layout requires a stencil with more than one level of apertures. This is often the case when a large copper pad is included underneath the device not only to provide a reliable electrical connection but also to dissipate heat during the reflow process. In these instances it is possible to create’step-up’ areas within the stencil which are used to increase the height and volume of the solder paste deposit in selected areas – useful for ‘pin in paste’ devices and large format parts.

Finally, the method of cleaning the stencil is vital in ensuring that it performs as expected. The use of chemicals which are too aggressive can cause damage to the stencil’s apertures while ‘wet’ cleaning which uses water to clean the stencil can leave deposits on the aperture walls that may hinder the transfer of solder paste to the PCB. Using a high-quality, non-toxic cleaner which is safe for both the stencil and the substrate will ensure that your finished product is free of defects.

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