AN10365 Datasheet, PDF (15/31 Pages) NXP Semiconductors – Surface mount reflow soldering

NXP Semiconductors
AN10365
Surface mount reflow soldering description
Another important factor is the aperture shape, that is, whether the aperture is rectangular,
trapezoidal, or otherwise. Paste release also depends - amongst others - on the loading
and speed of the squeegee, the board separation speed, the printing direction, and the
aperture orientation. In essence, all of these parameters must be adjusted so that all
solder paste deposits on one board, from the smallest to the largest, are printed properly.
Consequences of insufficient solder paste printing are usually open contacts or bad joints.
These may arise because:
• The solder paste deposit is not sufficiently high: components or their leads may not
make proper contact with the paste, resulting in open circuits or bad joints,
or
• There is insufficient solder volume for a proper solder joint, also resulting in open
circuits,
or
• The activator is used up rapidly in a small solder paste deposit, so that the paste no
longer properly wets the component metallization, also resulting in open circuits
A second important aspect in solder paste printing is smearing. If some solder paste
bleeds between the stencil and the board during one printing stroke, then the next board
may not fit tightly to the stencil, allowing more paste to bleed onto the bottom of the
stencil. Once this effect starts, it strengthens itself. As a result, the solder paste may
eventually form bridges that stretch from one paste deposit to the next. If a bridge is
narrow enough, it will snap open during reflow, as the volume of molten solder seeks to
attain minimum surface area. A wider bridge, however, may remain stable, resulting in a
short-circuit.
To achieve a difference in solder paste volumes on one board, it is possible to use a
stencil that has a different thickness at different locations. An example of this is the
step-stencil. This, however, is only recommended if there is no other solution.
Stencils are commonly made from Nickel; they may be either electro-formed or laser-cut
(preferred). Typical stencil thicknesses are given in Table 3.
Table 3. Typical stencil thickness
Semiconductor package pitch
 0.5 mm
0.4 mm to 0.5 mm
Stencil thickness
150 m
100 m to 125 m
A general rule is that the stencil apertures must be 25 m smaller than the solder lands,
on all sides. In other words, the solder paste lies 25 m inward from the solder land edge.
This usually results in stencil aperture dimensions that are 50 m smaller than the
corresponding solder land dimensions; see Figure 17.
This rule does not apply for a BGA; for a BGA the solder paste deposit is shown explicitly
in the PCB footprint specification. Although BGA balls and their solder pads are circular,
square stencil apertures are sometimes preferred for a BGA.
AN10365
Application note
All information provided in this document is subject to legal disclaimers.
Rev. 6 — 30 July 2012
© NXP B.V. 2012. All rights reserved.
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