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ESD11B
The following is taken from Application Note
AND8398/D ? Board Level Application Note for 0201
DSN2 Package.
Printed Circuit Board Solder Pad Design
Solder Mask Openings
Based on results of board mount testing, ON
Semiconductor’s recommended mounting pads and solder
mask opening are shown in Figure 6. Maximum acceptable
PCB mounting pads and solder mask opening are shown in
Solder
Mask
Overlay
Solderable
PCB
Figure 7.
Figure 6. Recommended Mounting Pattern
Figure 7. Maximum Recommended Mounting
Solder Mask
Two types of PCB solder mask openings commonly used
for surface mount leadless style packages are:
1. Non Solder Masked Defined (NSMD)
2. Solder Masked Defined (SMD)
The solder mask is pulled away from the solderable
metallization for NSMD pads, while the solder mask
overlaps the edge of the metallization for SMD pads as
shown in Figure 8. For SMD pads, the solder mask restricts
the flow of solder paste on the top of the metallization and
prevents the solder from flowing down the side of the metal
pad. This is different from the NSMD configuration where
the solder flows both across the top and down the sides of the
NSMD SMD
Figure 8. Comparison of NSMD vs. SMD Pads
Typically, NSMD pads are preferred over SMD pads. It is
easier to define and control the location and size of copper
pad verses the solder mask opening. This is because the
copper etch process capability has a tighter tolerance than
that of the solder mask process. NSMD pads also allow for
easier visual inspection of the solder fillet.
Many PCB designs include a solder mask web between
mounting pads to prevent solder bridging. For this package,
testing has shown that the solder mask web can cause
package tilting during the board mount process. Thus, a
solder mask web is not recommended.
PCB Solderable Metallization
There are currently three common solderable coatings
which are used for PCB surface mount devices- OSP,
ENiAu, and HASL.
The first coating consists of an Organic Solderability
Protectant (OSP) applied over the bare copper features. OSP
coating assists in reducing oxidation in order to preserve the
copper metallization for soldering. It allows for multiple
passes through reflow ovens without degradation of
solderability. The OSP coating is dissolved by the flux when
solder paste is applied to the metal features. Coating
thickness recommended by OSP manufacturers is between
0.25 and 0.35 microns.
The second coating is plated electroless nickel/immersion
gold over the copper pad. The thickness of the electroless
nickel layer is determined by the allowable internal material
stresses and the temperature excursions the board will be
subjected to throughout its lifetime. Even though the gold
metallization is typically a self-limiting process, the
thickness should be at least 0.05 m m thick, but not consist of
more than 5% of the overall solder volume. Excessive gold
in the solder joint can create gold embrittlement. This may
affect the reliability of the joint.
The third is a tin-lead coating, commonly called Hot Air
Solder Level (HASL). This type of PCB pad finish is not
recommended for this type packages. The major issue is the
inability to consistently control the amount of solder coating
applied to each pad. This results in dome-shaped pads of
various heights. As the industry moves to finer and finer
pitch, solder bridging between mounting pads becomes a
common problem when using this coating.
PCB metallization.
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