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When new packages came on the market, Seritronics set themselves a goal: the paste volume had to go through the openings more easily.
This problem does not arise with openings of a certain size, but comes about in cases of fine miniaturisation.
Calking involves creating a bevel of about 30 microns on both faces of the stencil.
This offers various advantages:
1. Filling and emptying of pads is made easier. The solder paste can flow in during filling and out during emptying more easily, without enclosing air, helped by the double funnel pad shape. This offers obvious advantages compared to pads with sharp edges that are not rounded.
2. Less friction along the sides of the pads. The gripping surface on the sides of the pad is greatly reduced, compared to solutions without calking, for the same stencil thickness. This reduces friction and facilitates subsequent flowing out.
By way of example, for a 200 x 1500 micron pad formed in a 150 micron thick stencil, the gripping surface is reduced by 35%.
3. Better stencil removal. Once printing has been completed, the stencil and the foil are raised to remove them from the paste deposit. Transverse play is inevitable during this movement, which tends to deform the deposit, especially if the pad has sharp edges.
The rounded bevel tends to leave the deposit unharmed and repair it where necessary.
4. Easier cleaning of the stencil:
routine manual or automatic cleaning of the stencil is made easier as the surface area to be cleaned is smaller than for stencils with traditional pads.
The bevel also prevents tearing away particles of the cleaning agent, which may contaminate the paste.
This phenomenon is more commonplace where sharp edges are involved.
5. Better process performance.
To reach the correct compromise between opening and thickness, experience suggests that their ratio must be equal to or greater than two.
6. Where printing is carried out using a rubber flood bar, the advantage to using a stencil with round edges rather than sharp edges appears to be clear. This results in less wear on the bar, which retains its original, correct profile longer.
7. Calking only removes the material in the pad, leaving the thickness of the stencil unaltered.
8. To complete the process, “Nickel-Plating” was launched and implemented on a large scale in 1998.
With this process nickel is deposited on the inner sides of the pads, further facilitating the passage of the paste. It also results in the entire stencil being harder.
9. Nickel-plating resulted in glossy surfaces, which might have caused dragging along rather than rolling of the paste at varying viscosity values.