Solder paste can be effectively designed into complex automatic handlers and processorsparticularly for high volume applications, it can be reflowed using a wide variety of techniques. Solder paste lends itself to being used in selective areas and at times in the process where other techniques such as wave soldering, are impossible, or other systems very expensive (such as solder iron). Solder paste lends itself to the use of a hierarchy of metallurgies (and melting points) to allow for "progressive" soldering. Solder paste is also being used to allow effective acreage utilization such as double sided attachment and multiple stacks. At least one manufacturer is combining double stack with three stack attachment--using solder paste. However this would have been impossible with the alternative systems and is attainable now easily and reliably with vapour condensation techniques--to be discussed subsequently. Solder paste is allowing major flexibility to achieve new space saving and reliability improvements.
1. Introduction/Past History
Of course solder has been used for a long-time--not only for mechanical joints but for electrical assemblies. The early solder pastes and creams were used for basic assemblies such as auto bodies and especially auto radiators. These were simple systems designed for rapid use and low cost. The level of sophistication was also low. The emphasis was on the word, "cheap."
Previously, pre-microelectronics and in the days of semiconductors, solder paste was relatively unknown. Solder preforms, coined solder and other simple mechanical applications were used. For example, solder would be punched from a strip or a basic preform was used and located on a terminal which was to be soldered to a header. This approach did not lend itself to the complex designs of microclectronics especially considering that on bare substrates or even printed components there are no sites per se for even temporary preform attachment.
In some of the early microelectronics assemblies, especially with side terminals, soldering was accomplished by dipping, In many cases, it was found that this technique did not provide consistent quality especially due to leaching potentials. The author has presented data concerning leaching mechanisms.
In the past eleven years, solder paste has made major improvements--primarily due to ever changing applications, reliability and cost requirements.
Eleven years ago, the solder pastes used in electronic assemblies were very simple. As such the pastes had to be stirred every few hours to minimise separation. There was no problem with protecting the solder powder against oxidation because often there was a thick layer of organic "goo" on top. The reflow systems were generally simple--hot plates, walking beam combined with heated track and IR systems. Unfortunately the IR systems were not well understood and the controls required were even less understood. In these early days, IR systems were either highly praised or highly avoided. What was being soldered? The answer, a wide variety of both passive and active devices. Active devices included flip chips in automotive assemblies and complex semiconductor circuits attached to TO 36 headers. Passive components include multilayer chip capacitors, and weld tabs.