TABLE OF CONTENTS In Chap. 5, we covered the reasons for heating in soldering. We also went into the details of preheating and the sensitivity of certain materials such as ceramics to the rate of temperature change. Such vital considerations are an integral part in selecting equipment.
Reflow soldering (Sec. 6.16) is a good example, where several methods can be used. The major difference between them is the rate of heat transfer and the controls that are possible. These are best learned from the heat profile, which is a measure of the temperature rise in the assembly as a function of time, and/or the specific heating zones in the equipment. Careful measurement and experimentation during the selection process is obviously necessary. This may have to be part of the equipment vendor's qualification. Equipment cost, unfortunately, is a poor guide to successful equipment selection.
When two surfaces are to be joined by solder, they should reach wetting temperature at the same time. In the case where one part reaches soldering temperatures ahead of the rest, the flux and solder will bond to it and run away from the cooler surfaces. This is best explained by a simple example:
Figure 6.1 a depicts an assembly consisting of two flat members, fluxed at the work area with a solder preform located at the desired solder joint location (before heating).
Figure 6.1: The effect of uneven heating. Applies to flux and preform,...
