NXP Semiconductors AN10365, Примечание к применению

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AN10365_3 © NXP B.V. 2008. All rights reserved.
Application note Rev. 03 — 22 April 2008 19 of 24
NXP Semiconductors
AN10365
Surface mount reflow soldering description
This, however, is not the case for packages with solder balls. If these packages are
soldered, the IC package solder balls and the solder paste both melt, to form a single joint.
Therefore, it is essential that the reflow temperature profile reaches a temperature that is
high enough for both the solder paste and the solder ball to melt and to form proper solder
joints.
SnPb needs a temperature of at least 215
°
C, but at least 235
°
C is required for most
Pb-free solders. There are four options:
•
SnPb balls are combined with a SnPb paste; the balls and paste form good joints at a
temperature of 215
°
C or more; this combination has been used for decades
•
SnPb balls are combined with a Pb-free paste; the paste requires a higher reflow
temperature of at least 235
°
C; the solder balls only need 215
°
C, so they will also
melt properly: OK
•
Pb-free solder balls are combined with a SnPb paste; in this case, only the Pb-free
balls require a higher reflow temperature, whereas the rest of the process does not;
therefore, this combination is not advised
•
Pb-free solder balls are combined with a Pb-free paste; now the paste requires the
same reflow temperature, and so do the solder balls: OK
The text above is summarized in Table 7 .
5. Inspection and repair
5.1 Inspection
In general, Pb-free solder is a little less successful at wetting than SnPb solders; SAC
fillets will have a larger contact angle between the fillet and the wetted surface. When
using Pb-free solder, this contact angle may typically be 20
°
to 30
°
. Notice the difference
between SnPb and Pb-free solder in Figure 18 : in the photograph on the left (SnPb), the
solder lands have been wetted completely. In the photograph on the right, however, the
solder has left part of the solder lands unwetted.
Table 7. Compatibility of ball and solder paste alloys, for packages with solder balls
Solder ball SnPb solder Pb-free solder
SnPb balls mature technology OK
Pb-free balls not advised OK