SOLDER PASTE SCREEN PRINTING
Screening solder paste onto the MIB is the first step of this assembly technique. The screen should produce deposition patterns approximately the same as that of the pad metallizations.
Solder paste must be chosen with particle sizes significantly
smaller than the screen mesh opening, in order to achieve effective and
Wet deposit thickness is a function of the solder volume
required by the solder joint, and is primarily determined by the screen
parameters, e.g., mesh, wire size, and emulsion thickness. Approximate print
thickness may be determined from mesh and emulsion buildup parameters by
application of the following formula:
Tw = (Tm x Ao) + Te
Tw = wet print thickness
Tm = mesh thickness
Ao = decimal equivalent of percent open area
Te = emulsion
Absolute values are dependent on printer parameters, such as
squeegee pressure and speed. Reflowed thickness is a function of the volume
percentage of volatile materials within the paste composition.
Variable mesh techniques have been developed to vary and
regulate various solder volumes deposited on the same MIB. They allow precise
control over the amount of solder deposited, coupled with an ability to
accommodate a variety of device sizes.
The solder paste is applied by being drawn across and through
the screen by a rubber or synthetic squeegee which should have a sharp edge.
The squeegee should be adjusted so that it contacts the screen at an angle of
60° - 90° For long screen life and good detail definition, the pressure on the
squeegee should be minimized. The screen and screen printer should be
carefully cleaned with solvents compatible with the screen emulsion
SOLDER STENCIL PRINTING
Patterned metal foil stencils may be used to enhance both the
solder volume deposited and its uniformity. Stencils are used on conventional
screen printing equipment, with the equipment adjusted for contact printing.
Squeegee pressure should be sufficient to provide a clean wiping action of
metal foil without dishing out paste from the etched pattern. The metal foil
stencils should be designed with opening aspect ratios (stencil opening
width/stencil thickness) greater than 2 in order to facilitate stencil
fabrication, provide paste deposits with good edge definition, and avoid
clinging of the solder paste to the stencil sidewalls. Since solder paste
tends to pull towards wettable MIB pads during reflow, the stencil openings
may be designed to be larger than the pads in order to provide additional
solder volume. However, spacing between adjacent deposits must be sufficient
to prevent solder bridging.
SOLDER PASTE DISPENSING
In the dispensing process, paste from a supply cartridge is
pneumatically or mechanically forced through a small-diameter tube or orifice
to cause a small discrete amount of paste to be deposited at the MIB pad.
Robotic dispensing enhances registration precision and decreases through-put
Solder preforms are used to create pillars which help to compensate for
thermal coefficient of expansion (TCE) mismatch between the MIB and device,
and provide sufficient space under the chip carrier to improve module
cleanability. Metallization on the LCC bottom is required. Solder preforms
also provide very precise control over the amounts of