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The Short Scoop: Improving Stencil Printing Results

As a continuation of my July 2014 column, this month I am providing possible answers to the frequently asked question, “Why am I getting poor printing results?”

There are a myriad of causes of poor print performance. The problem may stem simply from an inferior or worn-out stencil, which is typically the first place people focus when troubleshooting. However, the issue may also be caused by an improper aperture design or stencil thickness. Additionally, poor print performance might not be caused by the stencil itself, but rather an improper printer set-up, a non-optimal squeegee blade, or the rheology of the solder paste being used during processing.

To shed light on this month’s topic, I have compiled a list of the problems our users encounter and possible solutions..

To find the cause of the problem, it is helpful to break the stencil printing process into two phases. The first is the fill phase, when the aperture is filled with solder paste. The second phase is the transfer process, during which the paste is released from the aperture and transferred to the pad on the PCB.

PROBLEM: Insufficient solder volume transfer

Potential causes for insufficient solder transfer are often associated with: 

1. Rough aperture walls that cause poor solder paste release, particularly as aperture sizes decrease

This problem has become more prevalent as board densities have increased and component sizes have shrunk. It is one situation that is directly related to the fabrication of the stencil. Different stencil fabrication techniques, such as laser cutting and electroforming, yield different levels of aperture roughness. Before selecting the stencil, evaluate the type of layout and configuration of the board, the types of components you are working with, and the board application. Then find the type of stencil that will give the paste release to meet those needs. The easiest to find and least expensive is an off-the-shelf laser-cut stencil. However, for more stringent applications where components are close together and very small, you might have to get a chemetch, NiCut, or electroform stencil to get side walls that are smooth enough for proper solder transfer, and therefore volume.Read the full column here.Editor's Note: This column originally appeared in the September 2014 issue of SMT Magazine.