“Board layout” VS “Stencil”

Hi,

I frequently see two terms: “board layout” design and “stencil” design. The drawings of the two are very similar with some minor difference in pad dimensions.

Could anyone explain what the differences between the two concepts are?

Matt

stencil design is used basically for smd..
after placing the smd on the board the stencil is kept above it..later the solder material is spread on the stencil and the smd is soldered..

board design is suitaable for the components other than smd..like capacitor. headers, bergs, etc.

after the borad is designed .. the components are placed on it and then the board is made to move on a conveyer belt over the bucket of solder metal..due to surface tension the the solder stics on the required parts ..and soldering is completed..

Always design your stencil to the IPC-7351 standard, which is 1:1 stencil artwork, you design the stencil at the same time you design the board.
It is just a artwork that shows where the appertures are to be to deposit solder paste for the reflow of components, mainly surface mount devices, but also increasingly some through hole components such as connectors, it should also have fiducials on the artwork so these can be half etched into the screen for the printer to locate and cater for any misaligment of the screen in the printer.
The reason why you shpould use a 1:1 for the generic artwork is so your stencil manufacturer can compensate for stencil thickness and other parameters, especially if you use multi level stencils.
Stencils are probably the most critical component in the reflow line, where incorrect use, design, cleaning, squegee pressure or a whole host of other problems. So I would strongly recomend that you create an IPC-7351 standard artwork (all pads 1:1 , fiducials on the artwork) and work with your assembly department and screen manufacturer to alter the appertures to cater for you processes and component sizes used.
Have a look around this site:
Tecan - SMT Stencil Products
A selection of info from the web:
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Printed Circuit Design and Fab and Circuits Assembly October 2010
PCDFCA September 2011

Note the stencil is for printing solder paste...it is not related to selective soldering.

abuzarshaikh7 and marce,

Many thanks for the answer. I will look at IPC-7351 for requirements.

Matt