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 18 of 24
NXP Semiconductors
AN10365
Surface mount reflow soldering description
When reflow soldering, the peak temperature should never exceed the temperature at
which either the components or the board are damaged. The maximum peak temperature
for components is partially determined by their moisture sensitivity. For reflow soldering
with SnPb solder, the peak temperature should be higher than 215
°
C; when soldering
with SAC, the peak temperature should be higher than 235
°
C. Note that this usually
implies a smaller process window for Pb-free soldering, thus requiring tighter process
control.
The black lines in Figure 17 represent the actual temperature profiles for a number of
different spots on a board. The bottom black line represents the coldest spot, and the top
black line represents the hottest spot. The blue line represents the minimum peak
temperature, and the red line is the maximum peak temperature. At the top left, some
regions on the board are exposed to temperatures that are too high, exceeding MSL
qualification conditions. At the bottom left, some regions on the board are exposed to
temperatures that are too low, resulting in unreliable joints. At the right, all of the regions
on the board have peak temperatures that fall within the upper and lower limits.
Reflow may be done either in air or in nitrogen. In general, nitrogen should not be
necessary; in that case, air is preferred because of the lower cost. Reflow may be done in
convection reflow ovens, some of which have additional infrared heating. Furthermore,
using vapour phase reflow soldering can reduce temperature differences on a board.
4.4 Solder and terminal finish or solder ball compatibility
In selecting a solder paste, care must be taken that the solder is compatible with both the
board and the IC package finishes. When soldering leaded or leadless packages, all
package finishes may be freely combined with all solders; see
Table 6 .
Fig 17. Fitting both the hot and cold spots into the required peak temperature range
001aac845
thermal damage
unreliable joints
minimum peak temperature:
minimum solder joint temperature
maximum peak temperature:
MSL test temperature
Table 6. Compatibility of ball and solder paste alloys, for leaded or leadless packages
Terminal finish SnPb solder Pb-free solder
SnPb mature technology OK
Pb-free OK OK