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21. Soldering

21.1 Introduction to soldering surface mount packages

This text gives a very brief insight to a complex technology. A more in-depth account

of soldering ICs can be found in our Data Handbook IC26; Integrated Circuit

Packages (document order number 9398 652 90011).

There is no soldering method that is ideal for all surface mount IC packages. Wave

soldering is not always suitable for surface mount ICs, or for printed-circuit boards

with high population densities. In these situations reflow soldering is often used.

21.2 Reflow soldering

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 100 and 200 seconds depending on heating method.

Typical reflow peak temperatures range from 215 to 250 °C. The top-surface

temperature of the packages should preferable be kept below 230 °C.

21.3 Wave soldering

Conventional single wave soldering is not recommended for surface mount devices

(SMDs) or printed-circuit boards with a high component density, as solder bridging

and non-wetting can present major problems.

To overcome these problems the double-wave soldering method was specifically

developed.

If wave soldering is used the following conditions must be observed for optimal

results:

• Use a double-wave soldering method comprising a turbulent wave with high

upward pressure followed by a smooth laminar wave.

• For packages with leads on two sides and a pitch (e):

– larger than or equal to 1.27 mm, the footprint longitudinal axis is preferred to be

parallel to the transport direction of the printed-circuit board;

– smaller than 1.27 mm, the footprint longitudinal axis must be parallel to the

transport direction of the printed-circuit board.

The footprint must incorporate solder thieves at the downstream end.

• For packages with leads on four sides, the footprint must be placed at a 45° angle

to the transport direction of the printed-circuit board. The footprint 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.

9397 750 06856

Preliminary specification

Rev. 01 — 4 February 2000

© Philips Electronics N.V. 2000. All rights reserved.

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