Philips Semiconductors

PCA8565

Real time clock/calender

Product data

Rev. 01 — 31 March 2003

22 of 26

9397 750 10695

© Koninklijke Philips Electronics N.V. 2003. All rights reserved.

15. Soldering

15.1 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 can still be used for certain surface mount ICs, but it is not suitable for fine

pitch SMDs. In these situations reflow soldering is recommended.

15.2 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, convection or convection/infrared

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 preferably be kept:

°C for all the BGA packages and packages with a thickness ≥ 2.5mm

and packages with a thickness <2.5 mm and a volume

thick/large packages

called small/thin packages.

15.3 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:

upward pressure followed by a smooth laminar wave.

– 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.