68-ROW driver for dot-matrix STN LCD
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
This text gives a very brief insight to a complex technology. For more in-depth account of soldering ICs, please refer to
dedicated reference materials.
15.2 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, please
contact Avant for drypack information.
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 techniques exist for reflowing; for example, 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.
Preheating is necessary to dry the paste and evaporate the binding agent. Preheating duration: 45 minutes at 45 °C.
15.3 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.
If wave soldering cannot be avoided, 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.
15.4 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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data sheet (v3)