Selective soldering processes

The laser beam is focused directly on the surfaces to be joined and the solder depot. The localized energy input rapidly heats the partners and the solder, with ramps from 500 K/s to well in excess of 1000 K/s possible, depending on the masses to be heated and the power of the laser. 

The solder melts and wets the partner components. As soon as the laser is switched off the solder cools and solidifies quickly, producing a fine-grained microstructure in the solder. A pyrometer can be used to measure the temperature in the soldered joint so that laser power can be adjusted to produce a desired temperature profile. 

FIGURE 5.12 Schematic of simultaneous laser beam soldering of a two-terminal component 

Consequently, like other selective soldering processes, laser beam soldering is used when 

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A variation of wave soldering is the so-called selective soldering process. Instead of using a long wave to accommodate... 

Conventional solder procedures require the use of high-temperature thermoplastics. Other plastics can be processed using low-melting-point solders, conductive adhesives, or selective soldering methods. Available assembly and interconnection technologies as well as processes for the production of the circuit carriers must be carefully considered, selected, and possibly modified to the specific MID materials. 

Mixed Metals. Two good examples of the use of mixed metals are for panels that have been selectively solder-stripped and for panels that have had fingers or contacts plated with gold before the solder mask process. The surface preparation process must be selected based upon the most delicate of the metals on the panel. 

Dispensing processes include those for an adhesive, solder paste, or both (e.g., surface-mount wave or selective soldering). Because the adhesive is dispensed prior to the solder paste, it is possible to inspect for defects—incorrect dot volume, run-out on nearby solder pads or lands, and stringers—prior to dispensing solder paste. 

Generally, with the increased thickness and/or complexity of the circuit board as well as with the increased mix of components, the gap widens between the process requirements of the product and the process capabilities of the equipment or operator. There are two possible solutions to this discrepancy Either a capital investment can be made to upgrade the equipment (if available) to assemble the product successfully, or the primary assembly process must be limited to those components having similar geometries and thermal characteristics. The remaining devices are then treated as odd-form components and are soldered to the circuit board using secondary processes (e.g., selective soldering or manual assembly). 

There are ancillary measures that can be taken to enhance the capability of a soldering process. For example, in the case of manually soldering components to a thick circuit board, the latter may be preheated to augment the heat input of the soldering iron. Similar approaches have been used for selective soldering, particularly when both the component and the circuit board are thermally massive. 

Selection of the flux type depends on the solder alloy, metal finishes on the board and components, condition of the surfaces to be joined, the type of soldering process selected, required solder-joint attributes, and the intended final use of the assembly. 

The pallet, selective soldering pallet, or shield is a masking device meant to shield certain secondary (bottomside) components or features of the PWB during the wave soldering process. On its topside are a recessed nest for the PWB and retention features to hold the PWB within the nest. There are cutouts that expose those areas to be soldered to the flux and... 

Pb-free alloy selection and Pb-free soldering process development. 

The soldering and curing temperatures of the connection mediums differ. The composition of the solder alloy is the main factor determining the temperatures that have to be achieved in the soldering process to ensure that the solder melts in the peak phase and wets the contacts. Consequently, when selecting a connection medium or, to be more precise, a solder alloy for MID, it is very important to take into consideration the maximum temperature capability or the temperature resistance of the plastic. 

Is the selected material compliant with temperature-resistance requirements (use and (soldering) process) ... 

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