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ICT – RADIO COMMUNICATION

Solder mask variance and stencil invariance
Along with the solder stencil, the solder mask on the PCB (which
usually has a familiar green colour) defines where solder paste
should be applied to the PCB. A popular design approach involves
making the solder mask openings (apertures) slightly smaller than
the metal pads underneath (so-called solder mask defined pads).
Many PCBs use liquid photo-imageable (LPI) solder masks
that meet the IPC-SM-840 Class 3 standard. Depending on the
fabrication lot, vendor, etc., the solder mask apertures can vary in
size and be misaligned due to solder mask registration errors. An
example of a solder mask with varying apertures is shown in Figure
3 (highlighted with red rectangles).

Figure 5: Side view of a pin-to-solder-to-pad interface where (a) a nominal
solder mask aperture size creates a proper connection to the pin and (b) a
larger than nominal aperture causes a pin open (not to scale; dimensions
exaggerated for illustration purpose).

aperture size, while neighbouring pads have nominal or smaller
Figure 3: Examples (in red rectangles) of neighbouring LGA pads having than nominal apertures. Figure 5a shows an exaggerated side view
solder mask variation. of an LGA package sitting on a PCB after solder paste has been
applied. When the solder paste heats up, the solder ball that forms
Where pads are solder mask defined and where there is aperture- in the well that results from the solder mask opening, stands high
to-aperture variation, the effective size of the exposed copper pad enough to bond to the recessed pin. Figure 5b illustrates a case
will vary, even though the copper pad itself does not vary. This is where the solder mask opening is larger than normal. Here again, a
especially true when there are large sections of copper, usually solder ball forms inside the well. However, because the opening is
connected to ground, beneath the solder mask. larger, the height of the solder ball is reduced and does not bond to
Let’s focus next on the metal solder stencil. Variations in the size the recessed pin.
of openings in a solder stencil can result in varying amounts of solder
paste being applied to each pad. However, because solder stencils are PCB footprint design guidelines
generally laser cut from stainless steel sheets by machines capable of With the goal of minimising the effects of PCB and package
minimum cut widths of 0.05 mm or better, and aperture tolerances of variance, the following guidelines are recommended for reliable
0.013 mm or better, the practical variance from opening to opening LGA PCB assembly. These guidelines are applicable to LGA packages
is very small. These tight tolerances ensure uniform volume of solder with pins of dimensions 0.25 mm (square or rectangular), and with
paste being deposited onto the exposed copper of the PCB. An 0.5 mm pitch, and similar geometries.
example of a solder stencil is shown in Figure 4. • PCB pads should be 20% larger in each direction relative to the
The combination of the solder mask size variations and solder nominal LGA package pin dimensions, as shown in Figure 6. This
stencils with little or no size variation can set up a situation in which ends up being approximately 0.05 mm larger for packages with
PCB assembly is not reliable due to pin-to-pin shorts and/or pin opens. 0.25 mm square pins.
Open circuits can be caused by a single PCB pad having a larger
Oversizing the pad helps reduce the effects of registration errors.
• The PCB pad to solder mask opening should be a 1:1 ratio. This
is neither a solder mask defined, nor a non-solder mask defined
solution, but rather a hybrid solution.

Keeping the 1:1 ratio helps with having both enough solder for
proper connectivity and sufficient solder dam between the pads to
help prevent shorts.
• On the paste mask/solder stencil, break up the ground paddle
opening into several smaller openings, such that they are similar
in size to the pins on the part.

This evenly distributes solder over the large, exposed copper
sections of the PCB.
Figure 4: Example of a solder stencil. • Remove the silkscreen LGA outline.

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