Chapter 5 of our comprehensive guide to stencils has been developed to help you understand how to apply paste for optimum results.
5.0 Squeegee Blades
There are two main methods of applying solder paste to a circuit board using an SMT stencil printer: squeegee blade printing and enclosed head printing. This section focuses on the most common method – squeegee blade printing. With squeegee blade printing, three print parameters can typically be controlled, squeegee speed, blade angle and downward squeegee pressure. Excessive pressure can result in damaged stencils, coining and breaking of webbing between fine pitch apertures. Too little pressure can result in skips if the stencil is not wiped clean. To achieve the best results, the blades must possess flexibility to consistently deliver the expected paste volumes.
5.1 Squeegee blade types
There are two squeegee material types: rubber/polyurethane squeegees and metal squeegees.
Metal squeegees blades are most commonly used and operate at lower pressure than rubber variants. As such, they do not scoop paste from apertures, and because they are metallic, they do not wear as easily as rubber types – and hence do not need to be sharpened. The popularity of metal squeegees has grown with the prevalence of finer pitch components.
Metal squeegee blades are typically made from stainless steel in either a single thickness (0.150mm) configuration or can be manufactured with an etched mobile/recessed edge where the base material is 0.300mm thick and the edge is tapered down to 0.150mm.
5.1.1 Single-thickness metal squeegee blades
Single-thickness squeegee blades are laser-cut from specially selected 0.150mm annealed stainless steel and are ideal for applications that contain a mix of components. They offer a wide pressure window, excellent consistency and minimal wear.
5.1.2 Mobile edge metal squeegee blades
Mobile edge metal squeegee blades are etched from 0.300mm stainless steel with the edge feathered down to 0.150mm thickness. Blade flexibility and its reaction to under-stencil interruptions such as proud vias, poor silk-screening or solder resist imperfections can influence the success of the printing process.
Mobile edge blades adapt better than single thickness blades. In instances where a raised PCB feature is adjacent to fine pitch apertures it can create interruptions to the stencil flatness that cause the squeegee blade to “jump” or skip over an aperture leading to either retention of paste in the aperture, leading to an insufficient or possible bleeding and bridging. Mobile edge blades possess extra flexibility at the interface with the stencil surface and can adjust their profile, local to the under-stencil deformation, without sacrificing the print quality to achieve cleaner more consistent printed deposits.
5.2 Squeegee printing parameters
Printing with squeegee blades involves angle, speed and pressure.
5.2.1 Blade angle
Generally, the squeegee blade should be angled at 60° to the stencil surface. Sufficient downwards force should be exerted as the squeegee blade traverses the stencil, on the printing stroke, to ensure the paste rolls and fills the finest stencil apertures.
Decreasing the blade angle to 45° will provide extra downwards force to encourage increased aperture filling. Care should be taken to ensure the squeegee blade pressure is not excessive at this angle as it is possible to break the gasket seal between the stencil and the PCB resulting in bleeding, bridging and shorts.
The speed at which the squeegee moves across the stencil plays a large part in the effectiveness of the solder paste application by determining how much time is available for the solder paste to ‘roll’ into the stencil apertures and on to the PCB pads. Typically operators use a speed of 25 millimetres per second, but this can be adjusted depending on the size of the apertures and the type of solder paste being used.
Squeegee pressure depends mostly on the length of blade being used. Blade lengths should be the equivalent of the board width + 25mm at each end. If the squeegee blade employed is longer than this dimension it is often not possible to achieve good printed results without increasing the pressure significantly. As a rule of thumb 0.5-1.0 kgs / 50mm should satisfy most solder paste printing requirements. Beware of using excess pressure as inconsistent printed results and under-stencil contamination usually occur.
5.3 Squeegee condition and maintenance
Squeegees need to be carefully stored and maintained as any damage can lead to poor printing results. They should be checked before use and thoroughly cleaned after use, ideally using an automated cleaning system so that any solder paste residue is removed. If any damage is noticed to squeegees they should be replaced to ensure a reliable and repeatable process.
Squeegee life and quality should be discussed with all your process engineers and operations and should be established as a standard.
With stainless steel metal squeegee blades, extra care must be taken in handling. All squeegee blades should be inspected before installation and be replaced if they contain any edge flaws such as nicks or dents.
Operators should flag and stop any printer / squeegee setup which is not wiping properly. This is an indication of a squeegee or printer setup problem. The swipe of the squeegee should produce a smooth shiny stencil surface with no puddles or streaks of solder paste left behind.
5.4 Squeegee blades available from Tecan
5.4.1 Standard ranges
Tecan offers a range of off-the-shelf squeegee blades that are fully compatible with most leading SMT printer types. These can be purchased directly from www.stencils.co.uk