Philips Semiconductors
Product specification
Remote 8-bit I/O expander for I2C-bus
PCF8574
Several techniques exist for reflowing; for example,
13 SOLDERING
thermal conduction by heated belt. Dwell times vary
between 50 and 300 seconds depending on heating
method. Typical reflow temperatures range from
215 to 250 °C.
13.1 Introduction
There is no soldering method that is ideal for all IC
packages. Wave soldering is often preferred when
through-hole and surface mounted components are mixed
on one printed-circuit board. However, wave soldering is
not always suitable for surface mounted ICs, or for
printed-circuits with high population densities. In these
situations reflow soldering is often used.
Preheating is necessary to dry the paste and evaporate
the binding agent. Preheating duration: 45 minutes at
45 °C.
13.3.2 WAVE SOLDERING
This text gives a very brief insight to a complex technology.
A more in-depth account of soldering ICs can be found in
our “IC Package Databook” (order code 9398 652 90011).
Wave soldering is not recommended for SSOP packages.
This is because of the likelihood of solder bridging due to
closely-spaced leads and the possibility of incomplete
solder penetration in multi-lead devices.
13.2 DIP
If wave soldering cannot be avoided, the following
conditions must be observed:
13.2.1 SOLDERING BY DIPPING OR BY WAVE
The maximum permissible temperature of the solder is
260 °C; solder at this temperature must not be in contact
with the joint for more than 5 seconds. The total contact
time of successive solder waves must not exceed
5 seconds.
• A double-wave (a turbulent wave with high upward
pressure followed by a smooth laminar wave)
soldering technique should be used.
• The longitudinal axis of the package footprint must
be parallel to the solder flow and must incorporate
solder thieves at the downstream end.
The device may be mounted up to the seating plane, but
the temperature of the plastic body must not exceed the
specified maximum storage temperature (Tstg max). If the
printed-circuit board has been pre-heated, forced cooling
may be necessary immediately after soldering to keep the
temperature within the permissible limit.
Even with these conditions, only consider wave
soldering SSOP packages that have a body width of
4.4 mm, that is SSOP16 (SOT369-1) or
SSOP20 (SOT266-1).
During placement and before soldering, the package must
be fixed with a droplet of adhesive. The adhesive can be
applied by screen printing, pin transfer or syringe
dispensing. The package can be soldered after the
adhesive is cured.
13.2.2 REPAIRING SOLDERED JOINTS
Apply a low voltage soldering iron (less than 24 V) to the
lead(s) of the package, below the seating plane or not
more than 2 mm above it. If the temperature of the
soldering iron bit is less than 300 °C it may remain in
contact for up to 10 seconds. If the bit temperature is
between 300 and 400 °C, contact may be up to 5 seconds.
Maximum permissible solder temperature is 260 °C, and
maximum duration of package immersion in solder is
10 seconds, if cooled to less than 150 °C within
6 seconds. Typical dwell time is 4 seconds at 250 °C.
13.3 SO and SSOP
A mildly-activated flux will eliminate the need for removal
of corrosive residues in most applications.
13.3.1 REFLOW SOLDERING
Reflow soldering techniques are suitable for all SO and
SSOP packages.
13.3.3 REPAIRING SOLDERED JOINTS
Fix the component by first soldering two diagonally-
opposite end leads. Use only a low voltage soldering iron
(less than 24 V) applied to the flat part of the lead. Contact
time must be limited to 10 seconds at up to 300 °C. When
using a dedicated tool, all other leads can be soldered in
one operation within 2 to 5 seconds between
270 and 320 °C.
Reflow soldering requires solder paste (a suspension of
fine solder particles, flux and binding agent) to be applied
to the printed-circuit board by screen printing, stencilling or
pressure-syringe dispensing before package placement.
1997 Apr 02
19
NXP [ NXP ]
