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Code: EP39

TODAY, the wide variety of surface mounted components that have to be assembled on printed boards no longer permits the assumption that solder paste stencil apertures for reflow soldering can be the same size and shape as their related component footprint lands. They may need to be larger or smaller, depending on the component type, choice of stencil thickness and the gasketing method. It is also now evident that good land design and good stencil design are inseparable and should be carried out simultaneously. Those who regard themselves as professional engineers responsible for printed board and film circuit layout will recognise an interest and responsibility for including stencils in their job specification.

The aims are mainly cost-saving and product reliability — first, by significantly raising the chances of a ‘right first time’ stencil; secondly, by enabling high yield and maximum economy of solder paste usage to be achieved; thirdly, by reducing board area required, and fourthly, by responding to a growing reliability database. The majority of assemblers use far too much paste on their solder joints — up to 30% saving is practicable using the SM Club software. Most of the data that board designers require for this task are part of the information that they should already have obtained in order to do the board or substrate layout itself.

The three main elements of the design task are:

1. Selecting the best stencil thickness compromise to meet the often competing requirements of large and small components on the same board. For example, to achieve target fillet shape a large chip capacitor may need a 0.15 or a 0.2 mm thick stencil and a fine pitch integrated circuit lead or a 1005 (0402) chip may need a 0.1 mm thick one.
2. Designing the size, shape and location of each land and then each related stencil aperture to deliver the target quantity of paste. This involves eliminating excess wherever possible and minimising the effect of inevitable local shortages. Board size and the capability of the intended printing machine type, e.g., in setting and maintaining alignment accuracy, are relevant factors and details of its performance should be noted before starting the work.
3. Deciding whether the extra cost of a special (e.g., electro-formed or stepped) stencil is a necessary solution to overcome the combined effects of minimal copper thickness, solder resist thickness and/or the mix of component types on the printed board or substrate.

To make the work simple and quick, the author offers general guidance on stencil design plus a software package containing a series of linked spreadsheets. These cover three basic stencil thickness options (0.1, 0.15 and 0.2 mm) for chip components, gullwing (L-shaped) leads, J-leads and butt leads. Other stencil thicknesses and local thinning and plating methods can be factored in and the user may vary the target fillet height for chip components. The 12 spreadsheets are available in Microsoft Excel™ or Lotus 123™ for Windows 95.

In order to apply appropriate stencil aperture design detail and advice, the designer is given a choice of either using his/her own component and land size data inputs, applying the SM Club figures based on draft IEC 61188-5 component sizes, or using the original pre-stencil printing IPC/IEC standards. As the range of outlines within the so-called ‘component standards’ can vary by significant amounts — and so much depends on the precise dimensions of the specified items, the DIY option using the component manufacturer’s data sheets is by far the preferred one.

The output from the software informs on the proportion of the target solder joint volume or fillet height that has been achieved by the designer’s dimensional inputs on land and stencil aperture size. Within the constraints of the printing process and reliability issues, the input dimensions can then be varied, enabling the paste print to give a result that attains or approaches the target solder joint contour.

Important features of the software are its inbuilt capability for a quick check procedure for individual component problems and the fact that spreadsheets can be used in the ‘what if’ mode. These allow the user to identify the effects of varying the value of each of the component land and aperture dimensions on key aspects of the solder joint. Details of the solid geometry formulae and other assumptions are given so that the user can amend them at will. Automatic warnings are given when guideline design rules are violated. The system is suitable for both printed board and film circuit applications.

For technical details contact David Boswell: Tel/Fax autoswitch: 44 (0)1258-830302. e-mail: david@smclub1.demon co.uk