Take Advantage of Increased SMT Process Reliability with Mastercut Technologies Lasercut and Chemical Milled Stencils

Why Mastercut Technologies Stencils?

Accuracy

	Positional tolerance typically <15 microns across the print area
	Direct data use ensures the highest precision
	Mastercut Technologies offers you the state-of-the-art technology in the field of micro-cutting
	Minimise re-work for fine-pitch components

Quality
Smooth tapered aperture walls ensure easy release of the solder paste. All laser-cut stencils are electro-polished to give a wall roughness typically <3 microns.

Electro-polishing also passivates the stencil material, giving maximum protection from the elements while the stencil is in storage.

Mastercut Technologies laser-cut stencils maximise return by reducing printing errors, re-work and rejects.

Magnified view of a sectioned fine-pitched component

A non-electropolished YAG laser cut aperture (left),
is compared after electropolishing (right).
Even though the aspect ratio is small (1.2), the smooth,
tapered wall promotes good paste release.

Service

	Staff at Mastercut Technologies offer technical support for the supply of high quality stencils
	Direct data transfer by e-mail facilitates fast turnaround
	Standard delivery is 3 days
	Next day delivery is available

Technology Comparison

Laser Cut Stencils
Mastercut Technologies uses the most advanced stencil cutting technology developed by the German company, LPKF. The system has been specifically designed to manufacture stainless steel SMD solder paste and adhesive stencils. The result is a stencil of unsurpassed quality and accuracy, only comparable to one cut by another LPKF machine. The system is also capable of manufacturing encoder discs and precision thin metal parts.

Technical Data

	Resolution 0.5 µm
	Axial precision +/-10 µm
	Edge roughness typically < 3 µm

Mastercut Technologies's machine is set up to provide a small wall taper to assist in paste release. The taper, or draft angle, is typically 2° on each wall. To further assist in paste release, all of Mastercut Technologies's laser-cut stencils are electropolished. Stencils can be dispatched as plate only or we have a full framing facility.

Electropolishing
Electropolishing removes the micro-burrs associated with laser cutting and also passivates the steel, making it more resilient to corrosive cleaning agents. The result is smooth aperture walls, promoting a 'clean' transition of paste from the stencil to the PCB. Customers previously using significantly more expensive electroformed stencils have reported Mastercut Technologies's laser-cut and electropolished stencils to perform with no apparent difference in print quality on fine-pitch components.

Chemically Etched Stencils
Mastercut Technologies also manufactures chemically etched stencils for customers not requiring a laser cut stencil. This is a cheaper alternative for boards with no fine pitch components on them. Mastercut Technologies only manufactures stainless steel chemically etched stencils. The nature of chemical milling ensures smooth walls, with a rounding effect at the top and bottom edges, making the process excellent for printing boards with no fine pitch components on them., Mastercut Technologies is an Australian leader in manufacturing chemically etched precision thin metal parts and the process has been developed to produce chemically etched stencils of exceptional quality.

The illustrations below were produced using a scanning electron microscope.
Stencil produced by etching technology: Pad pitch: 300 microns

Stencil produced by laser technology: Pad pitch250 microns , pad aperture: 120 microns

Stencil Design

There are a number of important factors that will affect the printing performance of a stencil. These must be considered when designing and ordering a solder paste stencil.

These are:

1. The final aperture sizes. This determines the volume of solder paste that will be printed on the PCB. Three dimensions (on rectangular apertures), the length, width and the wall height of the aperture determine the volume of the solder brick printed on the board. The height, or stencil thickness, will have a significant effect on the performance of the stencil and subsequently the product defect rate. Correct specification will minimise product defects and rework. See below for more information. The final aperture sizes will also take into account any reductions required to be made. Reductions will also be discussed in more detail.
   
   
2. Whether the stencil obeys certain laws of physics that will guarantee successful transfer of solder paste from the stencil to the PCB. During printing, there exists an adhesion of the paste to the pad on the PCB and to the aperture walls of the stencil. The adhesion to the pad must be greater than the adhesion to the stencil to ensure a good transfer. Therefore it can be deduced that the printability will depend on a ratio of the stencil wall area to the open face area. This is of course ignoring other minor influences such as wall roughness and draft angle.
   
   
3. The type of stencil required. Laser-cut or Chemically etched. Mastercut Technologies recommends all stencils for boards with fine-pitch components (25 thou or less) should be laser-cut. The reliability, quality and fast turnaround times of laser-cut stencils make them the only choice for many of Mastercut Technologies's discerning customers.
   
   
4. Stencil dimensional accuracy and printing positional accuracy. Stencil dimensional accuracy is dependant on the quality of transferred cad data, methods and technology used to manufacture the stencil, and the conditions of use (i.e. the stencil is not being used at higher than normal temperatures). The printing positional accuracy will be determined by the alignment methods used.

Aperture Aspect and Area Ratios

When specifying stencil thickness it is important to look at the fine-pitch components. In general, the component with the smallest aperture will dictate the thickness of the stencil. If the thickness is specified so that the smallest aperture satisfies either the aspect or area ratio, then good paste release will be achieved for all apertures on the stencil.
Aspect Ratio: W /T >1.5
Area Ratio: LxW /2x(L+W)xT >0.66

The ratios given are generally accepted in the industry, however lower ratios are possible with smooth tapered aperture walls. The aspect ratio is generally used when the aperture length is 5 times or more than the aperture width. When the length equals the width (for square or circular apertures), the area ratio is a better indicator. The equations can be rearranged to give the thickness as a variable:
Aspect Ratio: T < W /1.5
Area Ratio: T < LxW /1.32x(L+W)

Aperture reductions and shape modifications It is beneficial and often necessary to make aperture reductions. We have extensive experience in making the correct reductions for different packages required on a stencil, and will be more than happy to assist customers in designing the most appropriate stencil for their requirements. We can also supply information on aperture shape modifications designed to eliminate solder balling under chip components.

Zelflex Stretching Frame
Stretching frames represent significant benefits in the SMT printing process. For high volume users, the frame represents a significant saving in stencil costs.
Other benefits

	Storage space required for stencils is significantly less than conventionally framed stencils.
	Faster turnaround times achievable.
	User-friendly stencil exchange and tensioning mechanism.
	Perfect and even tension with 4 sided pneumatic system.
	Stencil tension can be adjusted by regulating the supply air.
	Once the frame has been charged with compressed air, it can be disconnected and used, or left connected to the air.
	Can be adapted to fit most printing machines.
	Stencil cleaning is simplified.

The Zelflex Frame

We have had a number of opportunities to distribute various framing systems, however none have been what we would call an excellent system. The Zelflex is an excellent system and undoubtedly the best we have seen. Other systems have either taken too long to load, had too few attachment points making it unsuitable for thin stencils, or had too many problems with the politics of patents. Zelflex is an LPKF product. See their web site at www.lpkf.de