Christian Koenen at the EPP Innovations Forum

In the course of the miniaturisation of semiconductor components and the dynamic development of new technologies in the sector of microchip production, it is not only the component spectrum which is changing, but also the requirements of the circuit boards.

A brief look at the usual order options for circuit boards shows the variety of support materials on which they are subsequently printed and assembled. With rigid circuit boards various FR4 materials can be distinguished, and with flexible circuit boards frequently glue-free Polyimid materials are supplied. Often up to 16 copper layers are processed and the contacts are mostly done via microvias. With contact surfaces we differentiate between chemical Nickel Gold, HAL lead-free or chemical tin. Depending on the manufacturer, HDI microvia and Flex circuit boards can only be produced in connection with chemical Nickel Gold or chemical tin.

The upper surface consistency in particular has an influence on the stencil printing result. The current problems in component supply require creative solutions in order to manufacture complete component groups still more or less reliably with reasonable delivery times. Switching to alternative components and redesigning circuit board layouts are the order of the day. Quite frequently a huge time pressure situation arises periodically in the SMD production chain in order to satisfy customer requirements regarding quality and on-time delivery.

The quality of the stencil precision printing is substantially influenced by the circuit board consistency, the layout and the resulting topography. In this respect the topography of a circuit board does not mean the essential varying height of solder application of the various components. Rather more it concerns unevenness on and in the circuit board, for which new coordinated designs for the stencil will be needed. Always with the focus on perfect printing. The list of these topographical challenges is very long and includes topics such as:

Solder resist too high
Plugged Vias
Labels on the circuit board
Cavities in the circuit board
Parts projecting above the surface
Etc.…

These height differences can be from a few tiny micrometers up to several millimetres. Not infrequently a screen lift up is created by this and the seal of the stencil is impaired. Especially in fine pitch areas and with really thin stencil thicknesses solder leaks out of the press key area of the apertures and so leads to printing faults.

In particular with complex circuits in medium-size numbers the printing process can become unstable and faults such as “too little paste” or “short circuit due to too much paste” can be generated. The problem is that in this situation the repeatability of good printing results can become unpredictable. Often these strong fluctuations cannot be corrected by normal Pad design manipulations. However, printing faults can be avoided by targeted and precise stencil design. In addition to this, reproducible and secure paste release can be ensured for precision printing.

We would like to use this presentation to bring attention to this subject and show a few solution procedures on how to come as close as possible to perfect printing with topographical challenges on the circuit boards.

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Christian Koenen at the EPP Innovations Forum

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