Solder Printing Technologies : Solder Paste Printing Variables

3. Print variables

The following are some of the important print variables which are to be considered before any print cycle.

1. 3.1 Solder paste viscosity

2. 3.2 Print thickness

3. 3.3 Squeegee wear, Pressure, and hardness

4. 3.4 Print speed

3.1 Solder paste viscosity

Solder paste viscosity is very critical for obtaining an acceptable print. Smearing will occur if it is too low, and there will be skips if it is too high. Depending on the method of application as measured at 250C, the following viscosities are generally used.

1. Dispensing : 350000(+/- 50,000) cP

2. Screening : 500000(+/-50,000) cP

3. Stencilling : 650000(+/-1,00,000) cP

To minimize temperature induced viscosity changes, the paste should be printed in a temperature controlled area. In any event, since the viscosity of the paste will increase as some of the constituents evaporate, the paste should be removed from the stencil/screen at the end of each production run.

3.2 Print thickness

Print thickness determines the volume of the solder in the joints. Too thick a print will result in excessive solder joints or even solder bridges. Too thin a print will result in insufficient solder joints. The thickness of the paste print is determined by the thickness of the metal mask of the stencil (or the emulsion thickness for a screen). generally, the print obtained form a stencil 8 to 10 mil thick will be thick enough to generate acceptable solder joints on 50 mil lead pitch packages. For finer pitch (25 mil or less) packages, paste only about 6 mils thick is required. This requires use of multilevel thickness stencils.

3.3 Squeegee wear, Pressure, and hardness

The squeegee used to push the paste across the surface of the stencil at a speed that allows the paste to roll into and fill the stencil openings. The typical squeegee is a piece of hard plastic, such as polyurethane or Teflon, shaped in a rectangle. Metal squeegees made of steel are also used to achieve the desired hardness and uniformity. The squeegee is held mechanically with a fixture that assures secure grip at an even steady force. The squeegee force supplies the horizontal and vertical shear force necessary to push the paste across the stencil and down the openings. The force on the squeegee should be adjusted so as to produce a clean stencil surface on a single pass. Greater force results in excess paste scooping. Less force leaves uneven paste deposits and smearing on the lands due to paste bleed under the stencil. Offsetting the stencil from the surface of the board rather than allowing it to sit directly on the board helps improve the print definition and minimizes potential smear and bleed. This technique is called offset printing or off-contact printing. Its counterpart is direct contact printing, where the board and stencil are flush together.