Squeegee blades and lead-free solder paste

William E Coleman examines the effects of lead-free solder on squeegee blade performance.

With the implementation of lead-free solder paste there are questions regarding the impact on the overall printing process, such as the effect of lead-free solder paste on squeegee blade performance.

This study compares the performance of five different squeegee blades and clearly demonstrates that the AMTX E-Blade outperforms the rest of the field.

There are two critical characteristics of the squeegee blade that influence the overall print process performance.

One is the minimum squeegee pressure to achieve clean paste wipe on the squeegee side of the stencil during the print operation.

Excess solder paste left on the stencil surface after a print pass is bad for two reasons.

First, extra solder paste left on top of "large" apertures will get transferred to the PCB pad during the board stencil separation operation leaving excess solder paste volume on the "large" pads.

If the residue paste left on the stencil surface is not uniform, as is usually the case, different volumes of solder paste will be transferred to "large" pads across the PCB.

On the other hand, when excess solder paste is left on the stencil surface over "small" apertures the solder paste on the surface acts to hold onto the paste preventing complete transfer of the paste to the PCB pad.

Likewise, as the residue paste left on the stencil surface is somewhat random the volume of paste transferred for "small" apertures/pads will have paste volume variation.

Therefore it is very important to assure that the squeegee blade pressure is adjusted to a level to provide a clean wipe of the solder paste from the stencil surface during the print operation.

However adjusting the blade pressure to a "high" level to achieve the necessary "clean wipe" can have deleterious results.

Overly "high" blade pressure can result in a poor print results by printing in a "smear mode" rather that the preferred "shear mode".

In addition, squeegee pressures that are too "high" can cause damage to the stencil as well as the squeegee blade itself.

As a result, a good "figure of merit" for squeegee blades is one that will operate in the "clean wipe" mode at the lowest squeegee pressure.

The other common measurement used in evaluating squeegee blade performance is the amount of residue solder paste left on the squeegee blade after printing.

If too much paste sticks to the blade it can creep up onto the blade holder where the paste dries and eventually falls back into the paste reservoirs on the stencil.

These clumps of dried paste become lodged in stencil apertures causing blockage for that aperture.

Five different types of squeegee blades were tested: a nickel/Teflon coated blade, a nickel coated blade, the OEM squeegee supplied with the printer, a nickel titanium nitride coated device and the AMTX E-blade Electroformed squeegee blade.

All squeegee blades were 250mm long.

A DEK 265 GSX screen printer was used for all blades with the print speed set at 50mm/s.

The lead-free solder paste composition was 95.5% Sn, 4.0% Ag, 0.5% Cu with a viscosity of 990kcps and a solids content of 88.29%.

An AMTX E-FAB electroformed stencil was used for all the tests.

The minimum squeegee pressure was found by increasing the pressure by 0.5kgf per step until a clean wipe was achieved.

There was considerable variation in minimum squeegee blade pressure varying on the low end from 0.144kgf per centimetre of blade length for the AMTX E-Blade to 0.4kgf per centimetre for the nickel coated blade with a Teflon coating.

The next test was to measure the amount of residual solder paste left on the five squeegee blades after printing.

This was achieved by weighing the blade and holder before printing and measuring after printing with the residue solder paste left on the blade.

The amount of paste left on the blades varied from a low of 9.5g to a high of 17.1g.

The difference between the lowest and highest residue paste amounts to a little over 0.5cm3 of solder paste spread over the 250mm length of the squeegee blade.

In no case was there enough paste left on the blade to rise up onto the blade holder.

It was contained on the blade itself where it presents no problem to the print operation due to paste drying on the blade holder and falling back into the solder paste reservoirs on the stencil.

These data support claims in the industry that certain squeegee blades, namely the nickel/Teflon, retain less residue solder paste on the blade after printing for lead-free solder paste.

However, the difference in residual solder paste among all five blades tested was so insignificant that it was not a factor in squeegee blade performance.

A significant factor, as far as squeegee blade performance, is the differences in minimum pressure among the five squeegee blade types tested.

The AMTX E-Blade provides a clean wipe at almost one-third less pressure compared with the nickel/Teflon squeegee blade.

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