AN10365_3
© NXP B.V. 2008. All rights reserved.
Application note
Rev. 03 — 22 April 2008
10 of 24
NXP Semiconductors
AN10365
Surface mount reflow soldering description
2.3 Solder paste
In line with european legislation, it is recommended to use lead-free solder paste,
although exemptions are granted for selected applications, such as automotive.
A wide variety of Pb-free solder pastes is available, containing combinations of tin, copper,
antimony, silver, bismuth, indium, and other elements. The different types of Pb-free
solder pastes have a wide range of melting temperatures. Solders with a high melting
point may be more suitable for the automotive industry, whereas solders with a low melting
point can be used for soldering consumer IC packages.
As a substitute for SnPb solder, the most common Pb-free paste is SAC, which is a
combination of tin (Sn), silver (Ag), and copper (Cu). These three elements are usually in
the range of 3 % to 4 % of Ag and 0 % to 1 % of Cu, which is near eutectic. SAC typically
has a melting temperature of around 217
°
C, and it requires a reflow temperature of more
than 235
°
C.
[1]
Temperature is measured at solder joint.
A no-clean solder paste does not require cleaning after reflow soldering and is therefore
preferred, provided that this is possible within the process window. If a no-clean paste is
used, flux residues may be visible on the board after reflow.
For more information on the solder paste, please contact your solder paste supplier.
3.
Moisture sensitivity level and storage
If there is moisture trapped inside a package, and the package is exposed to a reflow
temperature profile, the moisture may turn into steam, which expands rapidly. This may
cause damage to the inside of the package (delamination), and it may even result in a
cracked IC package body (the popcorn effect). A package’s sensitivity to moisture, or
Moisture Sensitivity Level (MSL), depends on the package characteristics and on the
temperature it is exposed to during reflow soldering.
The MSL of IC packages can be determined through standardized tests in which the
packages are moisturized to a predetermined level and then exposed to a temperature
profile. Studies have shown that small and thin packages reach higher temperatures
during reflow than do larger packages. Therefore, small and thin packages must be
classified at higher reflow temperatures.
The temperatures that packages are exposed to are always measured at the top of the
package body.
Depending on the damage after this test, an MSL of 1 (not sensitive to moisture) to 6 (very
sensitive to moisture) is attached to the IC package. For every product, this MSL is given
on a packing label on the shipping box. Each package is rated at two temperatures, for
SnPb and Pb-free soldering conditions. An example of a packing label is given in
.
Table 1.
Typical solder paste characteristics
Solder
Melting temperature
Minimum peak reflow
temperature
SAC
217
°
C
235
°
C
SnPb
183
°
C
215
°
C
