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Selective soldering is the process of selectively
soldering Soldering (; ) is a process in which two or more items are joined by melting and putting a filler metal ( solder) into the joint, the filler metal having a lower melting point than the adjoining metal. Unlike welding, soldering does not in ...
components to
printed circuit boards A printed circuit board (PCB; also printed wiring board or PWB) is a medium used in electrical and electronic engineering to connect electronic components to one another in a controlled manner. It takes the form of a laminated sandwich struc ...
and molded modules that could be damaged by the heat of a reflow oven or wave soldering in a traditional
surface-mount technology Surface-mount technology (SMT), originally called planar mounting, is a method in which the electrical components are mounted directly onto the surface of a printed circuit board (PCB). An electrical component mounted in this manner is referred ...
(SMT) or
through-hole technology In electronics, through-hole technology (also spelled "thru-hole") is a manufacturing scheme in which leads on the components are inserted through holes drilled in printed circuit boards (PCB) and soldered to pads on the opposite side, either ...
assembly processes. This usually follows an SMT oven reflow process; parts to be selectively soldered are usually surrounded by parts that have been previously soldered in a surface-mount reflow process, and the selective-solder process must be sufficiently precise to avoid damaging them.


Processes

Assembly processes used in selective soldering include: * ''Selective aperture tooling over wave solder'': These tools mask off areas previously soldered in the SMT reflow soldering process, exposing only those areas to be selectively soldered in the tool's aperture or window. The tool and
printed circuit board A printed circuit board (PCB; also printed wiring board or PWB) is a medium used in electrical and electronic engineering to connect electronic components to one another in a controlled manner. It takes the form of a laminated sandwich str ...
(PCB) assembly are then passed over wave soldering equipment to complete the process. Each tool is specific to a PCB assembly. * ''Mass selective dip solder fountain'': A variant of selective-aperture soldering in which specialized tooling (with apertures to allow solder to be pumped through it) represent the areas to be soldered. The PCB is then presented over the selective-solder fountain; all selective soldering of the PCB is soldered simultaneously as the board is lowered into the solder fountain. Each tool is specific to a PCB assembly. * ''Miniature wave selective solder '': This typically uses a round miniature pumped solder wave, similar to the end of a pencil or crayon, to sequentially solder the PCB. The process is slower than the two previous methods, but more accurate. The PCB may be fixed, and the wave solder pot moved underneath the PCB; alternately, the PCB may be articulated over a fixed wave or solder bath to undergo the selective-soldering process. Unlike the first two examples, this process is toolless. * ''
Laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The firs ...
Selective Soldering System'': A new system, able to import CAD-based board layouts and use that data to position a laser to directly solder any point on the board. Its advantages are the elimination of
thermal stress In mechanics and thermodynamics, thermal stress is mechanical stress created by any change in temperature of a material. These stresses can lead to fracturing or plastic deformation depending on the other variables of heating, which include mat ...
, its non-contact quality, consistent high-quality solder joints and flexibility. Soldering time averages one second per joint;
stencil Stencilling produces an image or pattern on a surface, by applying pigment to a surface through an intermediate object, with designed holes in the intermediate object, to create a pattern or image on a surface, by allowing the pigment to reach ...
s and solder masks may be eliminated from the circuit board to reduce manufacturing costs. Less-common selective soldering processes include: * Hot-iron solder with wire-solder feed * Induction solder with paste-solder, solder-laden pads or preforms and hot gas (including
hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
), with a number of methods of presenting the solder Other selective soldering applications are non-electronic, such as lead-frame attachment to ceramic substrates, coil-lead attachment, SMT attachment (such as LEDs to PCBs) and
fire sprinkler A fire sprinkler or sprinkler head is the component of a fire sprinkler system that discharges water when the effects of a fire have been detected, such as when a predetermined temperature has been exceeded. Fire sprinklers are extensively used ...
s (where the fuse is low-temperature solder alloys). Regardless of the selective soldering equipment used, there are two types of selective flux applicators:
spray Spray or spraying commonly refer to: * Spray (liquid drop) ** Aerosol spray ** Blood spray ** Hair spray ** Nasal spray ** Pepper spray ** PAVA spray ** Road spray or tire spray, road debris kicked up from a vehicle tire ** Sea spray, refers t ...
and dropjet fluxers. The spray fluxer applies
atomized Atomization refers to breaking bonds in some substance to obtain its constituent atoms in gas phase. By extension, it also means separating something into fine particles, for example: process of breaking bulk liquids into small droplets. Atomizati ...
flux to a specific area, while the dropjet fluxer is more precise; the choice depends on the circumstances surrounding the soldering application.


Miniature wave selective solder fountain

The miniature wave selective solder fountain type is widely used, yielding good results if the PCB design and manufacturing process are optimized. Key requirements for selective fountain type soldering are: ;Process: * Nozzle diameter selection according to solder-joint geometry, nearby component clearance, component lead height and wettable or non-wettable nozzle * Solder temperature: Set value or actual value on plated through-hole part * Contact time * Preheating *
Flux Flux describes any effect that appears to pass or travel (whether it actually moves or not) through a surface or substance. Flux is a concept in applied mathematics and vector calculus which has many applications to physics. For transport ...
type: No-clean, organic-based; method of fluxing (spray or dropjet) * Soldering: Drag, dip or angle method ;Design: * Temperature requirement (for soldered part) and component selection * Nearby SMD through-hole component clearance * Ratio of component pin diameter to plated through-hole * Component lead length * Thermal decoupling * Solder masking (green masking) distance from component pad


Drop-Jet

The Drop-Jet is an Electromechanical device which is capable of depositing a droplet of flux on demand onto a surface such as a Printed Circuit Board and or component pin.


Thermal Profiling

The thermal profile of the selective process is critical as with other common automated soldering techniques. Topside temperature measurements within the pre-heat stage must be verified as with conventional flow solder machine, additionally flux activation must be verified as sufficient. As number of miniature profiling dataloggers are now available to make the process more simple such as the Solderstar Pro units.


Selective Solder Optimization

A number of fixtures are available to allow daily checking of the selective solder process, these instruments allow the verification of machine parameters to be performed on a periodic basis. Parameters such as contact time, X/Y speeds, nozzle wave height and profile temperature can all be measured.


Use of Nitrogen Atmosphere

Selective soldering is normally undertaken in a nitrogen atmosphere. This prevents oxidation of the fountain surface and results in better wetting. Less flux is needed with less left-over residue. The use of nitrogen results in clean, shiny joints without the need for PCB cleaning or brushing.


References

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