Structured CablingELCM 254

Lab 5Soldering

Created January 2012 for BXST

Goals:Solder a variety of joints using proper method.

Purpose:Create good solder joints using proper techniquesRecognize good and poor solder joint quality

Equipment Required:

Soldering irons, soldering kits (supplied) Variety of practice boards, components and lengths of wire (supplied)

Background information:

Solder

Solder is a metal alloy used to create a bond between metals. Its conductivity, low melting temperatures and stability over a range of temperatures makes solder the ideal choice for electrically interconnecting electronic components and wires.

Solder is an alloy (mixture of metals) which melts at temperatures considerably lower than the pure version of these metals. For comparison, the melting points of several materials are listed below:

Iron--------------------------------------------------------------------------2,795°FCopper----------------------------------------------------------------------1,981°FGold-------------------------------------------------------------------------1,981°FSilver------------------------------------------------------------------------1,761°FLead---------------------------------------------------------------------------621°FTin-----------------------------------------------------------------------------450°F63/37 (tin/lead) solder around----------------------------------------------361°F

Basic solder designed for use in electronic applications is made of tin and lead but due to concerns over the harmful effects of lead newer lead-free alloys have been developed. The European Union and other areas of the world comply with the RoHS (Restriction of Hazardous Substances) Directive, a standard that addresses lead-free solder. Presently there are over 100 different solder alloys available, selected based on application, durability, costs and other

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characteristics. The USA and Canada do not subscribe to RoHS however products meant for international sales may need to meet the directive.

Figure 1: Fusion characteristics of tin/lead solders.

Different ratios of lead (Pb) and Tin (Sn) will cause the alloy to have different temperature and behavioral characteristics. Tin/lead alloys may have a plastic phase between the liquid and solid states, a generally undesired characteristic in electronics soldering. Eutectic solder which does not have this plastic phase, has a ratio of 63% tin and 37% lead.

Solder is not meant to be used as glue; wires, circuit boards and leads should first be mechanically linked. Solder is meant to create a conductive bridge between components, filling in gaps and forming a stronger mechanical link. When solder comes in contact with a copper surface it actually dissolves a small amount of copper creating an inter-metallic bond. In some cases such as surface mounted components solder is used as both a conductive bond and a means of securing the component to the circuit board.

Figure 2: Molten solder dissolves and penetrates a clean copper surface, forming an intermetallic bond.

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Copper wire is frequently coated with tin during the manufacturing process. Tinning protects the copper against corrosion, maintains surface conductivity and eases the soldering process. Virtually all electronic component leads are tinned. It is important in a lead-free (RoHS-compliant) environment to select components that are tinned without lead.

Soldering Irons

A soldering iron is used to heat the component leads and wires, melting and bonding the solder in the process. Irons are application dependent; irons used for fine electronic work have different temperatures, grounding capabilities and tips than irons used for soldering large gauge wires or pipe.

A low voltage, temperature-adjustable, fine-tipped soldering iron is recommended for electronic work such as small gauge wire and circuit boards. Specialized finer-tipped irons are used for surface mount soldering. Irons used in electronics always have a base unit and a soldering wand. Soldering irons have interchangeable tips selected based on what is being soldered. The most common tips used in electronics are chisel-tipped with a conical shape.

A damp sponge is used to wipe the tip of the iron clean of solder and other residue. When storing the iron, a layer of solder should be left on the tip to protect it from oxidation. If a soldering tip becomes oxidized apply some clean solder to it, wipe it clean and apply a second, lighter coat of solder to it. Never leave a soldering iron on for extended periods of time without a protective coating of solder on the tip. Never use an abrasive on the tip to clean it.

Contamination and Flux

Copper and solder have a characteristic of rapid oxidation (‘rust”). Solder will not properly bond with oxidized or contaminated connections therefore it is extremely important that component leads, stripped wire ends, printed circuit boards, soldering iron tips and other connections are protected from contaminants such as:

oxidation oils (machine, skin) hand lotion smog silicones

Oxides and other contaminants interfere with the flow of solder and its ability to form an inter-metallic bond with the components being soldered. Flux is a chemical that is added to the joint to be soldered to help remove contaminants during the soldering process. Flux becomes acidic when heated which helps dissolve common contaminants, and boils violently when heated with the soldering iron, thereby scrubbing the contaminants from the surfaces. Flux creates a good solder joint by helping to distribute heat. Without flux, creating good solder joints would be

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very difficult. For this reason most electronics solder is comprised of a tube of metal encapsulating rosin, a solid form of flux. Proper hand soldering requires the addition of liquid flux to the joint.

The smoke that is often present when soldering is the flux or rosin burning away with the heat. Proper ventilation must be used when soldering as these chemicals are an irritant if inhaled.

The flux residue must be removed after soldering as the residues may affect the performance and longevity of the circuit. Flux also collects dust and other contaminants. Leaving flux residue is considered unprofessional.

The Soldering Process

A technician should have a good attitude and exercise good workmanship when soldering. Soldering is a technical art that requires practice to perfect. A good technician will be capable of reworking a board without leaving any evidence, with the only exception that the work done will often be better than the original assembly work.

The objective of a good joint is good wetting. Wetting is when liquid solder flows like a liquid, sticking to all parts of the metal joint. Like a liquid, melted solder will have some surface tension and will create a concave shape when filling in gaps. The characteristics of poor wetting include poor flow over the joint and a rough surface.

Characteristics of a good solder joint

All solder joints share common characteristics: Clean (no flux residue) Smooth Evenly distributed solder Shiny (note: some non-lead based solder may not be shiny) Able to see the lead outline Form a concave fillet

Figure 3: Cross-section of a round lead on a flat surface.

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Step-by-Step Soldering Process

The soldering process generally involves the following steps (note: there are different techniques used when soldering, depending on what is being soldered).

1- Shape and position the component lead or wire. Trim leads to approximately the diameter of the pad.

a. Component leads must be bent to the proper shape, perfectly centered in the hole.b. Wires must be mechanically linked where possible.

2- Clean the leads, circuit board pads or wires to be soldered using pure isopropyl alcohol or other suitable solvent

3- Apply soldering flux to the joint.4- Ensure the soldering iron is up to temperature

a. Set to 690 degrees F for most common wire and connector applications, 660 for most surface mount applications

5- Wipe the tip of the iron clean with a damp sponge.6- Apply a small amount of solder to the tip of the iron. This will help transfer the heat to

what is being soldered. This process is called “tinning the tip”.7- Apply the tinned side of the tip of the iron to the joint and hold it in place. About 1 to 2

seconds later apply the solder to the lead or wire, not the iron tip. The wire or lead should melt the solder.

a. Do not apply solder directly to the iron.b. Do not apply too much heat to the components for too long.

8- Remove the solder then the iron. Replace the iron into the holder without cleaning the tip.9- Inspect the joint for proper characteristics. 10- Clean and re-inspect the joint.

Figure 4: Cross-section of iron tip on a round lead. The X shows point of contact. Use of a solder bridge (right) increases the linkage area and speeds up the soldering process.

When components are heated with the soldering iron there is a real danger of component and circuit board damage. It is important to set the proper temperature on the iron, maintain a clean iron tip, use flux, apply minimal pressure and minimize the time on the component. Larger masses require more time to heat to the proper temperature. Heat generally rises and elements above the component are susceptible to heat damage. A third hand should be used to properly orient the component and a heat sinking clip should be placed between the iron and the component threatened by heat.

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Soldering Faults

There are many faults that may occur in soldering. The technician needs to be capable of recognizing the solder faults and know what corrective actions to take to eliminate these faults.

The most important rule when working with a circuit board is to protect the board. A circuit board that is damaged due to errors by the technician is inexcusable and potentially very expensive to replace. A technician must decide if a less-than-perfect solder joint is worth the risk of reworking and potentially damaging the circuit board. Proper training and experience will help minimize the risks to the circuit board. Use appropriate static protection, employ good soldering techniques and only use properly maintained soldering equipment.

The following is a list of terminology associated with soldering and soldering faults.

Birdcaging occurs when conductor strands are separated from their normal twisted configuration. The wires strands open up, leaving space between the individual strands.

Blowholes occur when hot gasses are released from below a joint just as the solder joint is hardening and are a problem with improper thru-hole soldering.

Bridges are when adjacent conductors or leads are connected with solder but are not meant to be connected. Poor lead preparation, poor circuit board design, excessive solder or poor wetting are the usual causes.

Cavities are seen when the solder has not properly filled the hole of a thru-hole joint.

Cold solder joints occur when the leads, wires or other items to be soldered are not brought up to melting temperature. Cold solder joints are caused by poor methodology and commonly occurs as a result of solder being applied to the tip of the iron and not to the joint. Characteristics of cold solder joints include poor solder flow, a dull and porous finish, poor mechanical connection and poor electrical conductivity as there is no inter-metallic bonding. Reflux and reflow will remedy this problem.

Contamination where a foreign material may interfere with the solder joint or with the function of a circuit has many possible sources. Examples include: melted insulating material interfacing with the solder, loose wire strands that may cause short circuits, adhesives in contact with joints, etc.

Disturbed joints occur when the objects are moved before the solder has hardened. A disturbed joint will have an uneven or somewhat dull finish with a wave shape indicating movement. A disturbed joint is weaker with poor electrical characteristics. Embrittlement occurs when the solder alloy creates a new, brittle allow when jointed with certain metals from the items to be soldered. This is common with soldering gold contacts. To

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Ideal Too much Not Enough

Board Board Board

Component lead

Solder

SolderSolder

No SolderComponent Side down

avoid embrittlement only use contacts designed for soldering. In some cases one may flush out the alloy through successive solder & solder removal processes.

Excessive wicking occurs when solder travels up the component lead and may interfere with its function. Especially problematic for connectors and switches. Excessive wicking may also occur with stranded conductors and cause it to become brittle.

Fractured joints occur when mechanical stress is applied to the solder joint after it has hardened. These are often caused by trimming leads after soldering, an unacceptable practice.

Icicles or projections are peaks of solder that are formed when the iron is removed from the work. Icicles are often seen when the solder is contaminated and when using non-eutectic solder where the solder is in a plastic state that tends to stick to the soldering iron tip. Icicles or projections may cause short circuits, violate clearance requirement or even injure someone.

A poorly formed lead may cause eventual failure of the part due to stresses placed on it (especially glass-bodies components such as diodes). Leads shouldn’t be bent any closer than 2 lead diameters from the component body. Leads must be centered and straight in the hole. Leads must be trimmed to a proper length. Add an extra bend in the component lead if it will be exposed to temperature variations.

A Lifted Pad occurs when the pad becomes separated from the circuit board. This condition is a result of too much heat, too many reworks, heat applied for too long, too much pressure applied to the pad, or a combination of these. Poor adhesion during board fabrication can also cause lifted pads. A lifted pad is considered disastrous.

Insufficient or excessive solder: a proper solder joint should have a proper fillet (concave shape), good flow and the leads should be visible. The pad should be covered with solder. This applies to the reverse side (component side) of a thru-hole board.

Figure 5: Examples of solder quantities

Scorching occurs when the soldering iron tip makes secondary contact with the component or conductor, or when the heat from the soldering process causes heat-related damage.

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Solder balls are a result of severe contamination where the solder could not stick to the joint and created a free-flowing ball created by the surface tension of the liquid solder. Solder balls may create short circuits.

Nicks and tool marks happen when improper tools or methodology causes physical damage to the lead or to the component. Not only does this appear unprofessional, but the component may exhibit different electronic behavior or fail over time.

Webbing or splashes are deposits of solder on a circuit board that are a result of solder dripping. The entire board must be carefully inspected for short circuits whenever a splash occurs.

Wetting is the fluid flow of solder. When solder is in a liquid state it will flow to fill the gaps and cover the parts being soldered. It has a natural surface tension that helps form the proper fillets in a good solder joint. Poor wetting or non-wetting is a condition where there was either insufficient heat to allow the liquid solder to flow naturally or there was too much contamination to allow the liquid solder to stick to the items being soldered.

Additional tools used with soldering

Long Nose Pliers with no grooves (small) and Lead Forming tool

Side (diagonal) Cutters or Flush (shear) Cutters(small)

Wire Strippers (small gauge)

Screwdriver Slot (Small) Others include Phillips, Robertson (green, red), and nut drivers

Third Hand, used to hold the items being soldered

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Heat Sink for Soldering used to absorb heat and reduce heat damage to components

Utility knife (standard) and metal ruler

Stereoscope and eye magnifier (loupe)

Soldering Station (adjustable temperature and interchangeable tips is desired)

Solder for Electronics (approx 20AWG 63/37 rosin) and solder wick fine

Brush, Flux bottle and 99% ISO Alcohol

Pointed Tweezers for surface mounted items and other fine work

Other tools include: static electricity dissipation systems heat shrink gun specialized tools for crimping connectors desoldering base station and systems tip conditioning compound masking tape

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etc…

Other Solder-related Terminology

Bifurcated Terminal: A terminal containing a slot or split in which wires or leads are placed before soldering.Breakout: In a laced wiring harness cable, a point at which a conductor or group of conductors exit from the laced harness or cable with sufficient stress relief.Bus Wire: A solid normally uninsulated wire or bar that serves as a common tie point for two or more electrical circuits.Cable: A group of conductors bound together by tying, lacing, or sleeving to convey electrical energy or signal from one point to another.Capillary action: The natural action of a liquid to move upward in a small tube due to surface tension. Applies to thru-hole joints in soldering.Certification: Documenting that personnel have completed required training, demonstrated specific proficiency, and have met other specified requirements.Conductor: A lead or wire, solid or stranded, or printed wiring path serving as an electrical interconnector between terminations.Conformal Coating: A thin, electrically nonconductive protective coating that conforms to the configuration of the covered assembly. Required in some applications, used to protect components and the circuit boards.Connection: An electrical termination.Encapsulating Compound: An electrically nonconductive compound used to completely enclose and fill in voids between electrical components or parts.Fatigue Fracture: A broken/disconnected solder joint due to vibration, temperature or other stresses. Problematic with surface-mounted components or with poorly formed and placed components.Land or Pad: A termination area on a printed circuit board used for electrical connection.Meniscus: Surface shape of a liquid created due to surface tension, either a concave or convex shape.Pierced (Perforated) Terminal: A terminal containing a hole through which leads or wires are placed before soldering.Plated Through Hole (PTH): A plated through hole is one formed by a deposition of metal on the inside surface of the hole. The configuration consists of a hole surrounded by a circular terminal area on each side of the board to provide electrical connection from one side of the terminal area, through the hole, to the other side of the terminal area.Potting Compound: An electrically non-conductive compound used to partially encapsulate, or for a filler between, parts, conductors, or assemblies. Resistance Soldering: Method of soldering by passing a current between two electrodes through the area to be soldered.Rework: The reprocessing of articles or materials that will make them conform to drawings, specifications, or procedures.Staking: Holding down a surface-mounted component using adhesives or other means.Strain Relief (component): Addition of bends in the leads to help absorb thermal and mechanical stresses.

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Solder Cup Terminal: A cylindrical holder to accommodate one or more conductors.Solder: A nonferrous fusible metallic alloy used to join metallic surfaces.Soldering: The process of joining metallic surfaces through the use of solder.Strapping Compound: An electrically nonconductive adhesive material used to anchor a part or component in place.Stress Relief: Method or means to minimize stresses to the soldered termination or part.Stud Termination: An upright conductor termination through a printed wiring board.Sub-Assembly: An assembly of one or more electrical/electronic parts that may be disassembled or separated without destruction of designed use; e.g., a printed wiring assembly.Terminal: A tie point device used for making electrical connections.Termination Area: A conductive surface on a printed wiring board used for making electrical connections (also referred to as a solder pad).Termination: The point at which electrical conductors are joined.Thermal Shunt (Heat Sink): A device with good head dissipation characteristics used to conduct heat away from an article being soldered.Timing: The coating of a surface with a uniform layer of solder.Turret Terminal: A round post type grooved stud around which conductors are fastened before soldering.Via or via hole: A connection between different sides or layers of a circuit board.Wetting: The fluid flow of liquid solder

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Types of Solder Joints for Wire

The following are some common joints associated with wire. Once properly shaped, each of these would be soldered.

1. Butt Joints

Joint where a wire meets another at a “T” intersection. This is a very weak joint.

The solder should flow smoothly onto the wire. Both wires must be heated equally to enable the solder to flow like water and produce a well-wetted joint. Soldering iron movement is important in establishing a good joint.

Figure 6: A butt joint

2. Western Union Splice

Western Union splices are good mechanical connections between two wires configured in series.

(a) Strip 1 to 3 inches of insulation from each wire.(b) Clean the wire.(c) Twist the ends of the wires tightly as shown in figures below.

Figure 7: Western Union splice

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3. Tap JointThis joint is made at an intermediate point along a length of wire. Carefully remove insulation from a middle section of wire and wrap the connecting wire around the exposed conductor. Process shown in the figure below.

Note: Wrap six turns.

Figure 8: Tap joint

4. Twist JointThis splice is used to connect wires that are parallel, whereas the Western Union splice is used to connect wires that are in line (series). The joint should be soldered for additional mechanical strength, protection, and better electrical connection.

Figure 9: Twist joint

5. Hook JointsSolid insulated to solid insulated wire (buss wire). Use the minimum amount of solder as shown.

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Figure 10: Hook joint

6. Stripping and Tinning of Stranded Insulated WireIt is important to tin wires correctly so that they can be bent around a terminal and make a good mechanical connection. Prevent solder from wicking in the wire underneath the insulation as this makes the wire solid and breaks easily. The strands of the wire should be visible through the solder.

Figure 11: Tinned wire

7. Hook TerminalsHook terminals are made with buss wire. Strip and tin the stranded wire only to the length needed to bend the wires as in the figures below. Keep insulation clearance equal to the wire diameter.

Figure 12: Hook terminal

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Figure 13: A small clamp and rubber band can be used to stabilize leads while soldering

8. Turret Terminal ConnectionsTurrets and terminals are used to create a good soldering platform for wires. Strip and tin wires before forming the leads on the terminal as shown in the figures below. All bases of the terminals should be tinned while soldering, but no solder spills should show on the body sides of the terminals.

Figure 14: Position of the wire on the terminal

Steps:

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(a) Apply iron to a point of maximum thermal mass.(b) Make a solder bridge to increase thermal linkage.(c) Apply solder to the side opposite the iron.(d) Remove the iron tip with a forward wiping motion.

See diagram, next page…

Figure 15: Steps in soldering conductors to turret terminals.

Insulation clearance should be equal to wire diameter when two wires are required on one terminal. They should be soldered at the same time, heating both wires and terminal at once. Since there is a large thermal mass more time may be needed on the terminal. A heat sink may need to be used to prevent the terminal from unsoldering from the circuit board pad.

9. Bifurcated Terminal SolderingStrip and tin wires. Bend the wires to the terminal to make a good mechanical

connection. When the wire lays on the base as shown in the figure below, the base must be tinned completely while soldering. No ends of wires must protrude outside the base. Strands of the wires must show after soldering.

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Figure 16: Position of the wire on a bifurcated terminal

Steps for soldering:(a) Apply iron to the point of maximum thermal mass.(b) Make a solder bridge to increase thermal linkage.(c) Apply solder to the side opposite the iron.(d) Remove the iron with a wiping motion

Seen next page for a diagram

.Figure 17: Steps for soldering conductors to bifurcated terminals.

Printed Circuit Board Soldering

Printed circuit boards come in a variety of types. For through-hole (thru-hole) components, the solder is always applied to the side of the board opposite that of the component; solder should never be applied from the component side.

With single-sided boards, thru-hole (leaded) components are placed on the side that has no pads and the leads are soldered from the opposite side. With double sided plated through-hole boards, solder must flow through the hole to the component side and form a proper fillet there.

The component leads must be trimmed prior to soldering. A general guideline is to trim the leads at roughly the diameter of the pad.

Most leads are left straight and should be positioned in the center of the hole. This will ensure solder flows and contacts all areas of the lead. In some cases the leads must be clinched (bent) to help keep the component in place. If clinching leads on solder pads with tracks, bend leads in the direction of track. If clinching leads on pads not connected to tracks, bend leads away from other tracks as shown in the figure below. All clinched leads should be cut at the border of the solder pad.

Clinching makes it difficult to rework a component so it is only used if necessary.

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Figure16: Types of circuit boards

Figure 17: Various lead positions. Note the leads in the holes: they must be centered and straight within the hole.

It is a good practice to install colour-coded resistors so that the colours are readable from left to right or from the bottom to the top of the boards. Other components must be placed consistently with printed information readable after installation.

Do not bend leads within 2 wire diameters of the component body because this can damage the component inside. Do not use sharp objects that can damage the leads. Component lead benders are recommended.

Figure 18: Typical solder system

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Pad

Solder

Component

Solder

Pad Pad

Solder

Surface Mounted Components and Circuit Boards

Surface mounted components are typically smaller than their thru-hole counterparts. Surface mounting significantly reduces the physical footprint of the circuit board.

Stereo microscopes, tweezers, small gauge solder and a good soldering system with a very fine tip are required for quality surface mount soldering. Temperatures on the iron are typically lower, and the time spent applying heat must be very limited.

Briefly here are the steps to successful surface mount soldering:1. Clean the pads and the components with iso alcohol.2. Apply flux to the pad3. Lay the fine solder across one of the pads. Using the pad as a measurement reference,

use the clean tip of the soldering iron to cut the solder and fill one pad. Reapply flux to that pad.

4. Using tweezers pick up the component from the sides. Hold it in position over the pad. Use the soldering iron to reflow the pad and allow the component to drop through the solder onto the pad and then position it. Do this quickly. Remove the heat.

5. Use the second pad as a measurement reference to cut and flow solder to affix the second contact of the component.

6. Inspect. The solder joint should have a concave fillet that extends from the top of the contact to the end of the pad. Other component leads, such as an extended “gull wing” lead from an IC, the solder should only extend to the top of the lead’s base.

When soldering other types of component leads the same general method applies. See diagrams below for proper solder joint on surface mounted components:

There are a variety of surface mounted components, leads, pads and other variables that are not addressed in this very brief introduction.

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Splice Board Project

Figure 19

Resistor thru-hole project

Figure 20

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IC Thru-hole project

Figure 21

Figure 22

©PRGodin and SAITJanuary 2012

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Lab Activity

The instructor will provide requirements for the lab activity and provide demonstrations as needed.

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