Basic Skills: Soldering

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BASIC SKILLS:  SOLDERING

by bd

INTRODUCTION AND ORIENTATION

Well developed soldering skills are fundamental to every automotive electrician.  The best electricians routinely solder electrical joints to create high-quality connections that deliver exceptional performance over the long term.  Based in applied technology, soldering skills are quickly learned through fast-paced orientation followed by methodic on-the-job practice.  Preliminary steps include surface preparation, sturdy mechanical binding between two or more conductors, selecting the solder, flux and method of heating appropriate to the specific task, and determining precisely where and how to position the heat source to safely access the joint without exposing the surrounding environment to damage or imparting personal injury.  Although the preparatory steps can consume several minutes to organize and complete effectively, the subsequent process of actually heating and soldering the joint, and then withdrawing the heat source at the precise moment the solder melts to allow the joint to cool naturally and undisturbed, consumes only a few seconds.  With moderate practice, the entire routine progresses quite rapidly with excellent results.

The following article reflects my experience commercially repairing vehicles and, occasionally, consumer electronics.  My purpose is to convey a simple procedure that can be applied to good results by everybody while providing sufficient details to satisfy people who have more insatiable curiosities.  Hopefully, my effort will answer more questions than it generates. 

I organized the topic into three essential sections:  on page 1, a fast-paced orientation to establish a broad foundation, on page 2, a step-by-step description of the applied technique, and on page 3, desoldering techniques.

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The Purpose of Solder:

• Circuit soldering is a process of emplacing highly conductive molten metal that when solidified encapsulates, seals and protects stationary electrical joints against the intrusion of gases, liquids, and dust that can oxidize and corrode connections creating unwanted circuit resistance that impedes current flow. 
  
  Proper soldering technique beneficially increases the effective volume (bulk) of a joint by filling the small open voids between mechanically bound conductors.   Essentially, the introduction of solder transforms the joint into a solid conductive mass, eliminating incidental resistance that otherwise might result from spotty, metal-to-metal contact (Fig. 1).  To lessor effect, solder benefits the transfer and dispersion of circuit generated heat for improved heat rejection and dissipation.
  
  
  

  Figure 1.  Mechanically bound conductor cross-sections that show: A) the effective contacts between bound only

  conductors, which manifest at the "points" illustrated across the arrows, versus B) the area of contact between

  bound and soldered conductors, which manifests as the solid conductive mass illustrated between the arrows.

Solder Selection:

• Solders used for routine electrical circuit repairs are two- or three-component alloys manufactured in precise component ratios.  Traditionally, the two most prevalent alloys are 60/40 (60% tin (Sn)/40% lead (Pb)) and 63/37 (63% Sn/37% Pb, aka, eutectic alloy) rosin core solders in wire diameters ranging between 0.018" - 0.062".  Less prevalent but equally suitable for electrical repairs is 70/30 (Sn/Pb) rosin core solder. 
  
  
• Each solder alloy varies in fusing temperature over a precisely defined range.  At the lowest end of each range, the solder just begins to melt and at the highest end, it is fully melted (Tbl. 1).  Of the various tin/lead alloy solders, only 63/37 eutectic alloy fully melts and solidifies at an exact point rather than progressively across a range of temperatures.
  
  
  
  Table 1 - Fusing Temperatures of Common Electrical Grade Solders
  
  

  SOLDER ALLOY % RATIO		SOLDER FUSING TEMPERATURE
  60 Sn/40 Pb	—	361 - 374 °F  (183 - 190 °C)
  63 Sn/37 Pb – eutectic alloy	—	361 °F  (183 °C)
  70 Sn/30 Pb	—	361 - 377 °F  (183 - 192 °C)

  
  
  
• Wire solder in diameters of 0.023" - 0.032" are better suited to soldering connections on printed circuit boards (PCBs) since less heating is required to melt the solder and the smaller diameter wire provides refined control.  For broad application general use, however, ~0.040" diameter wire solder is optimal.
  
  
• Molten solders possess a natural ability to wick along discrete, ultraclean metal surfaces that are packed together or very closely situated.  This beneficial quality helps give solder its capacity to permeate and fill voids in joints to the desired effect.
  
  
• Solidified solder turns electrical joints semirigid.  As such, solder is not suitable for connections that require flexibility.  For example, the wicking character of solder turns flexible stranded wire rigid for a short distance extending away from a soldered connection, locally limiting the flexibility of the wire.  The change in rigidity is quite abrupt at the terminus of solder migration along the wire.  With repeated high-angle flexing, a sudden change in wire rigidity can cause strands to break and separate inside the insulating jacket that encases the wire, causing troublesome opens.
  
  
• Though somewhat malleable at normal operating temperatures, solder can fracture brittlely if temperatures plummet, or otherwise fracture ductilely as a result of repeated thermal cycling or from low-cycle mechanical stresses (Fig. 2).  The susceptibility of solder to deformation fracturing makes solder, by itself, unsuitable for securing joints against mechanical separation that will result in physical breaks in electrical continuity.  Therefore, all connections to be soldered should be mechanically joined and effectively bound where possible, beforehand, such that they are self-supporting and cannot separate due to mechanical strain or subsequent fusing (melting of the solder) (Fig. 3).
  
  
  

  Figure 2.  Photomicrograph of a ductile fracture that propogates through a		Figure 3.  Routine mechanical binding of component leads prior to soldering.
  soldered joint, severing the connection.

  
  
  
• Due to environmental lobbies in recent decades, government legislation mandates lead-free solder substitutions in commercial electronics manufacture that blend tin, silver, copper (Cu) and antimony (Sb) in various combinations.  The inevitable trade-offs from eliminating lead from solder are greater initial cost (fueled in part by the mere fact of formal regulation), approximately 20% higher fusing temperatures, increased toxicity of vapors, accelerated erosion of soldering iron tips, and slightly heightened skill requirements.  Consequently, as long as they remain available, traditional electrical grade tin/lead solders are more practical and preferred for routine repairs.
  
  
• Environments that regularly encounter temperature extremes and/or repeated exposure to thermal cycling or standing wave vibrations (as evidenced by ductile fracturing of soldered connections - Fig. 4) may benefit from the use of 62/36/2 (Sn/Pb/Ag) or similar silver-bearing solder (up to about 7% silver) during repair service.  The drawbacks of employing silver-bearing solders routinely for all repairs are the significantly greater material costs incurred, the greater toxicity of flux vapors, and the introduction of slightly elevated melting temperatures that can cook thermally sensitive components.
  
  
  

  Figure 4.  Four typical examples of ductile failures occurring in PCB through-hole soldered joints that would benefit from repair using silver-bearing solder.

  
  
  
• The aluminum wire used to construct the rear lamp harnesses on some GM vehicles requires the use of relatively expensive, totally lead free, flux-cored tin/silver solder (96.5 Sn/3.5 Ag) that can be difficult to source.  Soldering aluminum wire requires exceptional surface preparation and ultraclean conductors.  Flux residues that remain after soldering should be thoroughly scrubbed away and the joint closely inspected to ensure proper fusing.
  

About Flux:

• Virtually all soldering fluxes are acidic electrolytes while active and are crucial to preparing conductors for metallurgical bonding.  Soldering flux effectively etches away oxide and contaminant coatings, chemically cleaning the conductors while inhibiting interim oxide formation as the joint heats. 
  
  
• Fluxes function as wetting agents that decrease the strong surface tension of molten solder to improve solder flow, enhancing the solder’s natural ability to wick.  When ample wetting occurs, molten solder locally mixes with copper atoms at the interface between the solder and conductors forming a microscopically thin, but completely new and unique, alloy layer and effective metallurgical bond.
  
  
• The introduction of flux is so important to the soldering process that insufficient application at the onset produces a "dry joint."   Dry joints result in poor chemical cleaning, which causes the solder to withdraw due to surface tension and not adhere.  Flux deficiency inhibits the solder’s ability to penetrate the joint and evenly coat and bond with the conductors.  Dry joints are visibly crude and inevitably produce poor connections in the long term, if not immediately (Fig. 5).  More often than not, the solution to a dry joint is simple resoldering using supplemental paste flux as necessary.  In rare cases, successful reworking may entail desoldering (removing the old solder), scrubbing the conductors to shiny metal, and then resoldering using supplemental paste flux.
  
  
  

  Figure 5.  Acceptable (left) versus dry (right) lead-free solder joints.		Figure 6.  Very dry solder joint.

  
  
  
• Flux naturally vaporizes and burns away as it approaches the melting temperature of solder providing short-lived benefit until depleted.  As the solder vaporizes and burns, it deposits a dark ashy residue.  If heating of a solder joint becomes prolonged, the resulting flux consumption and loss of flux action, exacerbated by the accumulation of ashy deposits, can create a 'very dry joint' (Fig. 6).  Hence, it is important to fuse the solder quickly while limiting the overall duration of heat application.
  
  
• Flux can be administered to a joint through various means, the most convenient and efficient of which incorporates paste flux within the core of the wire solder, administering flux to the joint automatically in appropriate proportion as the solder melts.  Other methods include various solder-free bulk pastes applied to the joints manually, just prior to heating and soldering (Fig. 7).  When purchasing paste soldering flux for electrical repairs, it is important to avoid specialized fluxes that are infused with solder to prevent inadvertent shorting across adjacent circuits in close proximity.
  
  
  

  Figure 7.  Rosin soldering flux bulk paste.

  
  
  
• Soldering fluxes fall into two primary categories, "rosin" and "acid," with variations of each.  The soldering of electrical circuit joints demands 'rosin' type flux.  Although all fluxes are acidic and therefore electrically conductive to varying degrees while active, rosin flux becomes chemically inert and nonconductive once it solidifies.  Unless an electrical environment operates near or above the melting temperature of the rosin flux, any flux residue that remains after soldering can be ignored and allowed to remain in situ.
  
  In contrast, 'acid' fluxes attract moisture and remain chemically active (caustic, corrosive and electrically conductive) indefinitely.  Acid fluxes etch conductors relentlessly and can sever current pathways allowed sufficient time.  In addition, they can produce bizarre symptoms by cross-circuiting adjacent current pathways.  Hence, introducing acid flux to any electrical environment is destructive and results in eventual circuit failure!  Consequently, the use of 'acid' flux in any form is NEVER appropriate for electrical applications!!
  

Heat Sources:

• There is a broad array of soldering irons and aids available, varying by purpose, price, physical configuration, convenience features, energy source, and temperature production.  With regard to electric irons, for any given configuration, the greater the power or wattage rating of an iron the greater the cost, the more robust the dimensions, weight and durability, the quicker the tip heats to operating temperature, the more effectively it transfers heat into the joint, and the hotter the tip becomes, typically in the range of 625 - 665 °F (329 - 352 °C) for low wattage irons, up to ~900 °F (482 °C) for 70-watt irons.  The rationale for such a broad spread between solder fusing and iron temperatures is the rapid rate of melting that ensues in an effort to minimize undesirable heat soaking of circuit components.
  
  
• Albeit, bigger is not necessarily better.  In confined spaces with limited access, a heat source possessing bulky physical size can present accessibility problems, jeopardizing ease of use, quality of the soldered joint, and safety.  If an iron consumes too much power for the application at hand, it can rapidly and unevenly overheat a joint, severely damaging the connected components.  Conversely, if an iron consumes too little power for the circumstances it will not transfer heat quickly enough to flash the joint and effectively melt the solder in a timely manner.  Instead of heat quickly transferring into the joint it will sink into the surrounding materials causing heat soak and thermal damage to insulation and temperature-sensitive components, while the metals to be soldered oxidize as the flux burns away.  Hence, soldering irons should be selected within the context of the specific tasks and ease of accessibility to the joints.
  
  
• Two common types of electrically powered irons that are well-tailored to soldering electrical joints are the compact, low-wattage pencil iron and the medium-wattage gun (Fig. 8).  A 7 — 35-watt nonadjustable pencil iron and a 5 — 60-watt adjustable soldering station are perfect for soldering discrete through-hole mounted electronic components that mount on a PCB via leads that protrude through the board from one side to the other.  A 100 — 260-watt gun producing up to 1100 °F (593 °C) is better suited for soldering uninsulated wire terminal ends, splices and joints up to ~8 gauge. 
  
  
• Cables in the range of 8 to 6 gauge fair better with the heat produced by a micro butane torch.  Battery cables in sizes 4 gauge and larger respond best when using a medium sized propane torch producing a neutral to slightly oxidizing (blue) tightly confined and focused flame.  The hazards in using an open flame heat source, besides the obvious potential for igniting flammable materials and causing personal injury, is the rapid oxidation of flux and copper elements, hindering solder wetting and adhesion, not-to-mention the annealing of copper elements.
  
  
  

  Figure 8.  Five common types of heat source:  A) pencil iron,  B) rudimentary soldering station,  C) gun,  D) butane micro-torch, and  E) propane torch.  Many options exist

  within the first three types, A, B and C.

  
  
  
• The best tip selections for electrical soldering are conical and chisel shapes.  This applies especially to pencil style irons that have many interchangeable tips from which to choose (Fig. 9).
  
  
  

  Figure 9.  Four examples of pencil style soldering iron tips:  A) knife,  B) conical,  C) oval,  and

  D) chisel.  There are many variations of each basic design.

  
  
  (continued below)
  
  

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SOLDERING TECHNIQUE

Preparation is a significant aspect of success and the dominant consumer of your productive time.  Rest assured that effort invested in methodic preparation will reap satisfying rewards when the job is completed and functioning as it should.  Although the following procedure emphasizes printed circuits (dominantly for the simplicity of illustration), there is little difference with regard to general wiring repairs.  The procedures are directly compatible and interchangeable.  To help minimize inhaling toxic vapors, take precautions to ensure the work area is open and well ventilated.  Always wear suitable eye protection!

• Mechanically remove and/or clean away lubricants, vulcanized insulation, paint, enamel coatings, surface oxidation, and any encrusted salts or other contaminants that may hinder the uniform flow and metallurgical bonding of the molten solder.  If surface coatings are hardened and stubborn, localized use of abrasives and/or systematic scraping to reveal shiny bare metal may be necessary.  For good results, metal surfaces to be soldered must be totally bare, free of all coatings and contaminants, bright and shiny.
  
  
• As discussed early-on in this article, whenever possible, create a tight, self-supporting mechanical joint between the conductors to be soldered that will tolerate vibration, moderate mechanical stresses, and cyclic heating-cooling (Fig. 3).  Conductors should be firmly bound together in such a way that if the joint subsequently heats to fusing temperature, melting the solder, the joint will not separate.
  
  Attaching wire and components to PCBs can be problematic, requiring creativity.  The thin (0.0002" - 0.004" (5 - 105 μm) thick) copper foils of PCBs are firmly affixed to their board surfaces but can be dislodged, spontaneously tearing free, if tension is exerted on them.  Because of this inherent weakness, with the exception of surface-mounted devices (SMD’s), wires and discrete component leads should pass completely through holes that are cleanly drilled through the PCB and then soldered to the foils on the side of the PCB that is opposite the side of lead insertion (Fig. 10).  So attached, any tension applied to the wires and/or components pulls the foils into the board rather than away.  Wires and the leads of large components that protrude through holes in a PCB can be bent to the side prior to soldering to improve mechanical binding (Fig. 11).  Clip the free ends of the leads close to the PCB after the solder congeals so that none extend further than about 1/8" (~3 mm).
  
  
  

  Figure 10.  Three methods of attaching flexible wires to a rigid PCB:  A) inserting the wire		Figure 11.  The options available for binding wire and component leads to PCBs can be somewhat limited and
  through the PCB and soldering it to the foil located on the opposite side of the board - the		problematic requiring simple creativity such as bending the leads that protrude through the PCB prior to
  PREFERRED METHOD, B) soldering the wire directly against the foil side of the PCB without		soldering.  Clip the free ends of the leads close to the board after the solder congeals.
  passing the wire through the board from the opposite side - POOR, and C) inserting the wire
  through the PCB, but from the foil side and then soldering - POOR.  The illadvised methods
  illustrated in B) and C) can subject the foils to tearing away from the PCB!

  
  
  
• Just prior to soldering electrical joints, preheat the iron to operating temperature and apply a sparing amount of rosin core solder directly to the tip of the iron (Figs. 12 and 13) OR momentarily dip the tip of the preheated iron into a container of rosin flux paste and then melt a sparing amount of wire solder directly onto the tip of the hot iron.  Immediately shake off or wipe away any dark ash and excess solder onto a wet cellulose sponge, being cautious of molten spatter that can cause personal injury, burn materials or impart cosmetic damage requiring subsequent first aid, cleanup and/or repair (Fig. 14).  This crucial step cleans and "tins" the tip of the iron, preparing it for an efficient transfer of heat into the joint.
  
  
  

  Figure 12.  Preheat the iron to full operating temperature.		Figure 13.  Generously 'tin' the tip of the iron.		Figure 14.  Remove ash, contaminants and excess solder from
  				the iron.

  
  
  
• Hold a pencil style soldering iron between your fingers using a relaxed grip as you would hold a pencil or pen.  After cleaning and tinning, lightly position the tip of the preheated iron directly against the joint and heat the bare, shiny conductors evenly (Fig. 15).  Apply a minuscule dab of solder directly to the tip of the hot iron while it rests against the joint to augment heat transfer into the joint, which significantly shortens the time required to bring the joint to melting temperature.  Immediately feed the solder into the side of the joint that is opposite the iron (Fig. 16).  Allow heat to transfer through the joint and into the solder, melting it.  The molten solder will naturally migrate through the joint toward the hot iron producing a uniform, penetrating bond.  NEVER feed solder directly onto a hot iron and allow it to drip or flood onto a joint!!  Without properly preheating the conductors, a "cold" joint will result that exhibits poor-to-no metallurgical bonding (Figs. 22 & 23).
  
  
  

  Figure 15.  Heat the joint.		Figure 16.  Apply solder to the joint.		Figure 17.  Upon fusing with good joint penetration, retract the
  				solder and iron simultaneously.

  
  
  
• To guard against inadvertent heat soak and associated thermal damage, stop applying solder and simultaneously withdraw the iron from the joint as soon as the solder wicks into and permeates the joint (Fig. 17).  Apply only the minimum amount of heat necessary to quickly melt the solder, allowing it to wick evenly into and around the clean conductors of the joint for effective permeation and adhesion.  Caution: if heat is excessive or applied for too long, severe thermal damage can occur to wire insulation, electronic components and the PCB as heat sinks into the conductors and adjoining materials.  Typically, the process of heating and soldering should consume no more than 3 - 4 seconds per joint. 
  
  
• Never move, disturb or artificially quench molten solder once emplaced!  To do so disrupts joint structure (i.e., the development of interlocking crystals, uniform grain packing and grain alignment), which weakens joint integrity producing a frosted appearance sometimes described as "crystallization."  To avoid solder crystallization, upon withdrawing the iron, allow the joint to cool naturally and undisturbed. 
  
  
• Highlights of the nearly identical procedure of soldering with a gun are illustrated in Figures 18 and 19.  Hold a gun style soldering iron as you would hold a pistol.
  
  
  

  Figure 18.  Prepping the gun for soldering.

  
  
  

  Figure 19.  Heating and soldering the joint.

  
  
  
• A properly soldered joint will appear bright and shiny with a smooth finish tapering at an angle of 40° ― 70° with regard to the PCB surface (Fig. 20).  The solidified solder of a good joint adheres to all of the metal surfaces onto which it flowed, spreading and tapering for a gradual, smooth transition onto the conductor surfaces (Fig. 21).
  
  
  

  Figure 20. Characteristics of an ideal solder joint.		Figure 21.  Good lead-based solder joint exhibiting a bright shiny
  		finish and smooth transitions.

  
  
  If the margins of the solder pull away from the conductors of the joint leaving gaps, either the components of the joint were insufficiently preheated resulting in a "cold joint" (i.e., poor metallurgical contact and bond between the solder and the joint - Figs. 22 & 23) or the joint was starved for flux and solidified "dry" resulting in poor electrical contact and no metallurgical bonding (Figs. 5 & 6).  Either scenario requires correction by reheating the joint to wick away the existing and/or excess solder followed by a sparing reapplication of fresh rosin flux and electrical grade solder.
  
  
  

  Figure 22.  Good versus cold solder joints.		Figure 23.  Cold solder joint.

  
  
  If the margins of the solder are bulbous and curve back under the leading edges, suggesting the appearance of a spherical ball resting on a flat plate, excess solder was applied to the joint with no place for it to flow (Fig. 24).  Although excess solder in and of itself may be harmless, it is unsightly and can hide significant errors such as cold joints, poor component lead protrusion, or even bridge narrow gaps between adjacent circuits.  Excess solder demonstrates poor technique and should be corrected by reheating and drawing away the excess, or by desoldering and resoldering.
  
  
  

  Figure 24.  Excess solder applied to a joint.  Note the lack of consistency between the

  applications of solder to the three adjacent joints in the foreground reflecting an error

  in method.

  
  
  (continued below)
  
  

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DESOLDERING TECHNIQUES

Now, for the seedy side of soldering skill: Desoldering!  The ability to take joints apart, cleanly and without damage, is as necessary a skill as putting them together.  It is inevitable that at some point you will encounter a soldered joint that needs to be disassembled to some degree, reconfigured, or completely undone and redone.  Such as it is, the process can be tedious depending on accessibility, the complexity of the joint, your level of experience, and your confidence in your abilities.

Disassembling a joint is notoriously more difficult than putting it together.  That’s just a fact.  There is little-to-no elegance involved.  Instances occur in which two or more methods must be combined for success.  As with soldering, an over-application of heat can impart disastrous consequences to thermally sensitive components.  Accordingly, conscious effort should be focused on minimizing prolonged component exposures to solder fusing temperatures (i.e., 361 °F (183 °C) or greater). 

As always, safeguard your eyesight by wearing safety goggles!


Wicking Technique:

• The simplest desoldering method wicks the bulk of the existing solder away from the joint using only an iron and supplemental flux.  The process relies on the fact that electrical grade solder is naturally drawn toward high heat as the solder melts.
  
  Preheat and tin the iron as you normally would.  Position the iron at the lowest point of the soldered joint.  As the solder melts, it will wick toward and onto the tip of the iron assisted by gravity.  To enhance the desoldering process making it more efficient, supplement the soldered joint with a sparing amount of fresh rosin flux by infusing additional flux-cored solder or a little rosin paste flux, wetting the joint.  As the tip of the iron floods with solder from the joint, shake or wipe away the excess from the tip, re-tin, and then reheat the joint, repeating until the bulk of the old solder has wicked away.  Finally, while still heating the joint pull or gently pry the lead(s) free, separating the bond.
  
  
• One variation of the preceding technique involves the integration of desoldering wick into the process.  Desoldering wick consists of a lightweight handheld spool of finely braided thin copper strands (Fig. 25).  The technique relies on the natural wicking property of solder. 
  
  
  

  Figure 25.  One example of desoldering wick.		Figure 26.  Desoldering wick inserted between the preheated iron and the joint.

  
  
  The use of wick is fairly straightforward but requires that the copper strands are not contaminated or oxidized.  Typically, the wick is pressed against the joint using the tip of the iron while the spool is held between one’s fingers.  Since the wick becomes quite hot during the desoldering process, maintain a separation of at least 2" - 3" between your fingers and the joint. 
  
  As always, preheat and tin the tip of the iron.  Position the free end of the wick directly against the soldered joint and heat the joint through the wick (Fig. 26).  As the solder melts it will wick into the braid drawing solder away from the joint.  If the copper wick is not in pristine condition, applying a sparing amount of supplemental bulk rosin flux paste to the joint and the wick before heating may improve the outcome.  Upon completion, cut any used wick that is saturated with solder from the end of the spool and discard it.
  

Suction Technique:

• If many joints are involved, desoldering is made significantly easier by applying momentary suction to the joint, withdrawing the solder as it melts.  This method is especially applicable to PCBs but works reasonably well with many types of soldered joints.  Various special purpose tools can be employed that introduce a sudden pinpoint pulse of very high suction to the joint, withdrawing molten solder instantly and cleanly without incidental spatter (Fig. 27).  Tinning a special purpose desoldering iron (Fig. 27A) is both unnecessary and undesirable because the solder does not adhere to its tip.  However, when using unheated solder suckers (Figs. 27B & 27C), a properly tinned soldering iron becomes necessary.  Simply position the tips of the preheated iron and suction tool against the solder joint, heat the joint to fusing temperature and then trigger the vacuum pulse.  The technique is painless and effective!  The only drawback is the modest cost of, yet, another tool.
  
  
  

  Figure 27.  Special purpose desoldering tools.  A) low to moderate wattage desoldering iron with a nickel plated tip, B) spring loaded, button triggered,

  solder sucker with a Teflon tip, and C) standalone rubber bulb with a Teflon tip.  All of the tool tips resist solder adhesion.

Low Pressure Forced Air Technique:

• Desoldering and separating wrapped joints (Fig. 3) can be particularly problematic, sometimes demanding a combination of techniques.  The initial step requires removing as much solder from the joint as possible through wicking and/or suction.  If the joint subsequently proves stubborn and reluctant to separate via careful mechanical prying of the leads, additional residual solder can be forced from the joint using short bursts of low pressure compressed air.  Directed forced air bursts can be applied using the rubber bulb of a desoldering iron (Fig. 27A), a standalone rubber bulb (Fig. 27C), canned air such as used for keyboard dusting, focused puffs directed from your mouth, or an ultra low pressure (<3 PSI) air hose and nozzle.  But, be warned!  Dislodging molten solder using compressed air is a last resort method that produces an uncontrolled dispersal of hot solder spatters requiring extensive cleanup!
  
  🚫 Using low pressure forced air to dislodge molten solder from a joint is hazardous, messy and often accompanied by consequences that need to be resolved, thus, should NEVER be employed without careful consideration and planning beforehand!  Wearing a suitable full face shield and redundant eye protection while keeping others away, at a safe distance, are paramount!  ALWAYS evaluate the surrounding environment for potential consequences and/or injury!  Bear in mind that molten solder manifests a temperature of >361 °F (>183 °C).  Although solder quenches rapidly, molten spatter can instantly cause painful and serious eye injuries that include blindness, impart cosmetic damage to surrounding materials, and create unseen short circuits.  The technique of using forced air to desolder a joint should always invite pause!