Component Solderability

The melting points of mixtures and solutions depend on the nature and the relative amount of each component of the solution. They are, however, lower than those of the separate components. Solder, for example, an alloy of tin and lead, melts at 183°C, a much lower temperature than either of its components tin melts at 231 °C and lead, at 328°C. 

This shows that he does not appreciate the difference between a drawing of some physical components soldered together (= mechanical connection) which does not lead to a device that has any electrical meaning as no current can flow and an electrical circuit diagram in which connections may coincide with physical connections (soldering points), but may as well be conceptual. 

Electronic components soldered into a printed circuit board (PCB). The bladi squares are microprocessor silicon chips, the central component of most electronics devices. (Arno Massee/Photo Researchers, Inc.)... 

In an ideal world, the transition from SnPb to Pb-free could happen overnight. However, in reality, the transition is not likely to happen instantly instead, this is expected to occur over a period of at least 6 to 12 months. During this time, SnPb and lead-free alloys are likely to coexist on the same printed circuit board assembly (PCBA), which poses potential issues with component soldering and reliabiUty. 

Continuous flexing of a PBA will fracture component leads and, more important, surface-mounted component solder joints, due to mechanical fatigue failure. (Mechanically induced flexing or vibration in assembled PBs is used under controlled conditions to induce failures in solder joints for quality and reliability studies.)... 

Thickness. When PCBs had all through hole components, solder mask thickness was not of much concern. However, with the advent of surface-mount technology in the 1980s, excessive solder mask thickness caused assembly problems, including ... 

Black film on solder. This condition can result when the solder alloy is high in tin. If solder reflow or component soldering is to follow, activate by tin immersion or with solder brighteners. Adjust phosphoric acad content in etchant and solder-plating conditions. 

When a lead, component, or PCB is immersed into solder during a solderability test, the following can be derived from the component solder interface Equation 42.1... 

Cell-phones, iPODs, digitial cameras, laptops and desktop computers, DVDs, and indeed all electronic equipment in our world have one thing in common They each contain an electronic assembly composed of integrated circuits and discrete components soldered to a printed wiring board (PWB) to create an electrically functional circuit. 

The board requiring repair is shielded, except in the area of component replacement, and positioned over the solder fountain. The shaped solder fountain is directed at the repair site. Once the component solder joints go molten, the component can be pulled out with gloved hand or phers, some flux can be added, and a new component can be inserted to complete the rework within one thermal excursion. Care must be taken to ensure that all solder joints have gone molten on the component being reworked. If not, PTH barrels can be ripped out of the board or barrel annuli can be damaged. 

Component solderability, which includes the solderability of the PCB, is probably the most important single characteristic to consider when building PCBAs. To be successful in soldering PCBAs, good wetting must be achieved. It is an ongoing effort to ensure that all boards and electrical components purchased and received from suppliers maintain good solderabhity. 

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