Assembly process Hand soldering

Lead-tin alloys are the most commonly used solder materials for microelectronic packaging. Pb and Pb-rich alloys have one of the most desirable characteristics of C4 solders in that they are soft and compliant. The compliant nature of these alloys act a cushion for absorbing thermal and mechanical stresses and transfer minimum stresses to the die during microprocessor assembly processes. On the other hand, the melting... 

The introduction of Pb-free technology has not changed, per se, available electronics assembly processes (reflow, wave, hand soldering, etc.). However, it has caused manufacturing engineers to reassess the parameters used in those processes because of two factors (1) higher process... 

The need for higher processing temperatures limits the assembly process window of Pb-free solders. A higher nominal temperature is needed to accommodate the temperature variation at components across a circuit board to ensure melting of the solder and adequate wetting and spreading at each interconnection. On the other hand, the maximum temperature must be limited to prevent thermal damage to heat-sensitive devices and the circuit board. 

Hand soldering may be achieved in an assembly (or slide ) line process for larger volumes. In this case, each one of several operators solders only a few of the total components of the circuit board. In the case of single-person work cells, the operator may solder all of the components to the circuit board. Alternatively, the work cell may be used to add odd-form components as the final assembly step on a nearly complete circuit board. 

As noted previously, placement defects can be identified in the machine by automated inspection. Alternatively, such defects may be detected by a separate machine inspection station or through visual inspection. A trade-off must be made between the number of parts inspected and detail of that inspection on the one hand, and the assembly process flow on the other. The more information that must be processed by the inspection step, the slower the placement process. Finally, every attempt is made to correct placement defects prior to the soldering step. 

Understanding the stresses to which the assembly process exposes the PCB is fundamental in defining the acceptability criteria. Process elements such as leaded or lead-free, reflow or wave solder, double-sided reflow, hand-soldering, and rework all significantly impact the requirements of the PCB. 

Generalized time and temperature parameters have been identified for a number of Pb-Sn solder assembly processes. Those parameters are listed below for reflow furnace soldering, vapor phase soldering, wave soldering, and hand soldering. 

Prior to 1950, these industries were based on vacuum tube technology, and most electronic gear was assembled on metal chassis with mechanical attachment, soldering, and hand wiring. All the components of pretransistor electronic products—vacuum tubes, capacitors, inductors, and resistors— were manufactured by mechanical processes. A rapid evolution occurred after the invention of the transistor and the monolithic integrated circuit. Today s electronic equipment is filled with integrated circuits, interconnection boards, and other devices that are all manufactured by chemical processes. The medium used for the transmission of information and data over dis-... 

Since they are so easy to rework, acrylics are also susceptible to inadvertent chemical attack from solvent splash during hand cleaning of solder joints elsewhere in the assembly. The high emission of solvent inherent in the acrylic process makes them less environmentally friendly than other materials. As the solvent evaporates, the coating shrinks and exerts stress on the components, so acrylics may not be suitable for all low-temperature applications. 

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