Philips SAA7345 User Manual download pdf (Page 35)

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.

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Philips SAA7345 User Manual Page 35

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