1998 Feb 16
35
Philips Semiconductors
Product specification
CMOS digital decoding IC with RAM for
Compact Disc
SAA7345
SOLDERING
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.
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).
Reflow soldering
Reflow soldering techniques are suitable for all QFP
packages.
The choice of heating method may be influenced by larger
plastic QFP packages (44 leads, or more). If infrared or
vapour phase heating is used and the large packages are
not absolutely dry (less than 0.1% moisture content by
weight), vaporization of the small amount of moisture in
them can cause cracking of the plastic body. For more
information, refer to the Drypack chapter in our
“Quality
Reference Handbook” (order code 9397 750 00192).
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.
Several methods exist for reflowing; for example,
infrared/convection heating in a conveyor type oven.
Throughput times (preheating, soldering and cooling) vary
between 50 and 300 seconds depending on heating
method. Typical reflow peak temperatures range from
215 to 250
°
C.
Wave soldering
Wave soldering is not recommended for QFP 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.
CAUTION
Wave soldering is NOT applicable for all QFP
packages with a pitch (e) equal or less than 0.5 mm.
If wave soldering cannot be avoided, for QFP
packages with a pitch (e) larger than 0.5 mm, the
following conditions must be observed:
•
A double-wave (a turbulent wave with high upward
pressure followed by a smooth laminar wave)
soldering technique should be used.
•
The footprint must be at an angle of 45
°
to the board
direction and must incorporate solder thieves
downstream and at the side corners.
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.
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.
A mildly-activated flux will eliminate the need for removal
of corrosive residues in most applications.
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.
