Electronics

Electrical Soldering

Alton G. Dunn

Laurens Central School

Abc’s of Electrical Soldering, by Louis M. Dezettel

Published by Howard W. Sams & Co., 1973.

Objectives

• Define what soldering is and explain what it does

• Describe the solder alloy

• Distinguish between organic and inorganic fluxes

• Identify types of soldering irons and take proper care of tips

• Use a soldering iron properly

• Observe safety precautions

Objectives

• Define what soldering is and explain what it does

• Describe the solder alloy

• Distinguish between organic and inorganic fluxes

• Identify types of soldering irons and take proper care of tips

• Use a soldering iron properly

• Observe safety precautions

What Soldering Is and Does

• Ancient soldering– 4,000 year old vases– Roman water pipes made of lead– “Plumbing” comes from Latin word

for lead (plumbum)

• Recent uses– Tin can

• Precise alloy of tin and lead

– Solder alloys and fluxes for plumbing and electronics

Electronic Soldering

What Soldering Is and Does

• Soldering– Joining of metals by heat for the

purpose of making something structural or producing a continuous and permanent path for the flow of electricity

• Joining metals– Soldering– Brazing– Welding

Joining Metals

• Soldering– Low melting point alloy (<800°F)– Heated to a liquid state– Flows onto the base metal– Alloys with it at the surface

• Brazing– Higher temperature– Different alloys and fluxes

• Welding– Base metals are fused– No separate alloy

What is a Metal?

• Free electrons in outer shell

• Interchange easily with free electrons of adjacent atoms– Makes good electrical conductors as

well as heat conductors

What is a Metal?

• Electrical attraction between negative electrons and positive ions makes metals solid

• Voids in crystal lattice structure• Alloys are combinations (mixtures) of

different metals

Soldering

• Solder is an alloy of lead and tin– Melts at a low temperature– Transfers thermal activity by

conduction to the surface of other metals (called the base metal)

• Combines with the base metal at the interface– Edges of the crystal lattice structure

overlap each other and join to form a solid that is mechanically and electrically continuous

What Can Be Soldered?

• Only certain metals are atomically structured to accept wetting with tin-lead alloy solder– Copper– Silver– Iron and steel– Zinc– Nickel– Gold– Platinum– Palladium

What Can Be Soldered?

• Other metals need special alloys and fluxes– Aluminum– Stainless steel– Magnesium

• Some are almost impossible to solder– Chromium– Cobalt– Silicon

Solder Alloy

• Solder– Alloy of two or more metals which, in

various combinations, is used to join base metals together by the application of heat

• “Soft solder” for electrical connections

• Tin (Sn, Stannum)• Lead (Pb, Plumbum)• Melting point depends on ratio

Solder Alloy

• Tin– Is not attacked by air and water– Reacts and alloys with the joining

metals well– But has too high a melting point by

itself– Also, too expensive

Solder Alloy

• Lead– Soft and dense– Reduces melting temperature– Adds to overall strength although it

moderates the action of alloying– Surface corrodes quickly

Tin-Lead Ratio

• 70/30– Good for pre-tinning; expensive

• 63/37 (best solder)– Eutectic solder; no pasty state

• 60/40– Good general purpose solder

• 50/50– Lower cost general purpose solder

• 40/60– Lowest cost common solder

ASTM* Specifications

• First two digits represent the percent of tin

• Letter suffix shows amount of antimony (used for hardening)– A = 0.12 to 0.25 percent– B = 0.5 percent– C = 2.0 percent

*American Society for Testing and Materials

Solder Additives

• Small amounts of one or more other metals that have special characteristics– Antimony

• Hardens the solder

– Copper• Reduces pitting of copper tips

– Silver or gold• Retards reduction of precious metals

from plated connections

– Cadmium• Lowers solder temperature

Tin-Lead Temperature Graph

• Pasty range

• Eutectic ratio

• Solidus (361°F)– Horizontal line representing

temperature below which solder will solidify as it cools

– Same for all ratios

• Liquidus– Minimum temperature for melting– Not the same for all ratios

Tin-Lead Temperature Graph

Solder Qualities

• Wetting– Good capillary action in flowing over

the base metal, bonding or alloying with its surface

• Temperature– Able to reach the liquid state and wet

the base metal at a low temperature

• Strength– As hard as possible consistent with

other qualities

Soldering

• Wetting– Solution action between the solder

and the base metal– Capillary action that breaks the

surface tension and flows solder along the surface of a metal

– Metal must be clean• Molten solder dropped on a nonmetallic

or unclean metal will form a ball (“bead”)

• Flux– Used with solder to remove surface

oxides

Wetting

Forms of Solder

• Rosin-core solders– Continuous inner core of flux– Some multi-cored

• As many as five cores of flux

• Available without flux core for use with liquid flux

Flux

• Oxygen in the air attacks copper, and forms a thin layer of copper oxide

• Flux acts on the oxides that form on the surfaces of metals– Thin tarnishes become soluble in the

flux, and evaporate when the flux is heated to its boiling point

• No new air can reach the metal, so it stays untarnished

Flux

• Organic “rosin” or “resin” fluxes– Noncorrosive– Used in electrical soldering– Charring at high temperatures

• Inorganic fluxes– Corrosive– More active than organic fluxes– Used in mechanical soldering

• Activated flux– Amine hydrochloride or other

ingredients added to organic flux

Flux

• Flux removes thin tarnish resulting form exposure of a base metal to air for a short period

• Flux will not clean– Greasy or oily surfaces– Scales of other corrosive

atmospheres– Oils and acids of human perspiration

Flux

Flux

• Dip and wave soldering of printed circuit boards (PCBs)– Flux applied in advance– Entire board goes into a molten bath

of solder– Rapid soldering system– Post-cleaning necessary if activated

flux is used

Cleanliness

• Do not touch component leads– Handle parts by their bodies– Handle printed circuit boards by the

edges, like you should a photograph or a DVD

• Clean old leads with emery cloth or fine sandpaper– NASA recommends a scraper tool

Forms of Solder

• Size– 1 lb. spools (also 5 and 20 lbs.)– Diameter depends on size of

soldering connection to be made

Wire Size Diameter

1/16 inch 0.062 in.

1/32 inch 0.031 in.

No. 16 0.064 in.

No. 18 0.048 in.

No. 20 0.036 in.

Length of 1 lb. of Solder Wire

Gauge Dia.

(in.)

Tin-Lead Ratio

60/40 50/50 40/60

18 0.048 182 ft. 174 ft. 167 ft.

20 0.036 324 ft. 319 ft. 299 ft.

22 0.028 536 ft. 529 ft. 493 ft.

24 0.022 865 ft.

26 0.018 1290 ft.

28 0.015 1910 ft.

30 0.012 2730 ft.

32 0.011 3590 ft.

34 0.009 4950 ft.

Soldering Irons

• Pencil-style soldering iron

Soldering Irons

• Pencil-style soldering iron

• Resistance soldering– Electrodes pass current through the

connection

Soldering Irons

• Soldering gun– Fast heating, trigger-pull– Specially treated, low-voltage copper

coil near the tip

Soldering Irons

• Heat control– Fixed wattage (W = E2÷R)

• Resistance increases with temperature• Dissipation of heat to the surrounding

air

– Self-regulating• Bimetallic strip or magnetic switch

– Transformer isolated• Dial adjusts voltage supplied to heating

element

Soldering Irons

• Transformer isolated– Dial adjusts voltage supplied to

heating element

How Much Heat Do You Need?

• Too high a temperature– Can damage semiconductor

components– Lift copper from a PCB– Melt insulation on a wire– Char the flux

• Too low a temperature– Take too long to transfer heat from

the terminal to the solder and melt it– Heat may affect attached parts

How Much Heat Do You Need?

• Wattage ratings of small pencil irons with approximate temperature reached at the tip

Wattage

(W)

Temperature

(°F)

25 650 – 700

40 750 – 800

50 850 – 1,000

Heat Recovery

Heating Elements

• Handles have sockets for interchangeable heating elements and/or tips

Tips

Care of Irons

• Tinning– Always keep a thin layer of bright

solder on the tip– Wipe excess solder on a damp

sponge– When idle, the tip should have extra

solder on it, or the normally thin layer will oxidize off

• Never leave idle without the tip installed

• Never hit iron against work bench

Care of Irons

• Retinning– Never retin a tip while it is hot– File the surface smooth

• Copper only, not iron or nickel clad

– Get burrs off with fine sandpaper or emery cloth

– Plug in for 60 to 90 seconds, then apply solder at the coolest tip temperature possible

– Discard tip if the end becomes too short from filing or if iron is pitted

How to Solder

• The main purpose of soldering is NOT to make a mechanical connection, but to make a strong electrical connection

• The purpose of the soldering iron is NOT to melt the solder!– It heats the connection– The base metal then melts the

solder

How to Hold the Soldering Iron

• Large soldering iron– Grip with your four fingers around

the handle, and thumb at the cord end, with the iron pointing down

• Small soldering iron– Hold a miniature style soldering iron

as you would a pen or pencil

Steps of Soldering

1. Apply the tip of the iron to the wire and terminal

– Don’t be afraid to apply a little pressure

– Object is to transfer heat from the iron to the connection

– Hold the iron there for two or three seconds

Steps of Soldering

2. Touch the end of the solder to the terminal right at the point where the iron is in contact with the terminal

– Solder should begin to melt in less than one second

– Feed about one inch of solder into the connection

– Let it flow into all the crevices between the wire and the terminal

– This takes about two seconds

Steps of Soldering

3. Remove the solder, then the iron– Don’t disturb the connection until it

has cooled– Should be a strong connection with

a bright appearance– All crevices filled, but with the

contour of the wire showing under the solder

– Should feather out, without rounded edges

*NASA says remove both at the same time

*

“Five America”

• Apply the soldering iron and count out loud, “One America, two America, Three America”

• Then while holding the iron to the connection, apply the solder and count on, “four America, five America.”

“Cold” Solder Joints

Wire Stripping

• Tools for stripping insulation– Production– Manual

• Remove about 1/8 inch more than the length to be soldered– If the insulation ends too near the

connection, some of the plastic could melt into the connection and prevent good wetting by the solder

– If it is too far away, it may short against other wires nearby

Wire Stripping

Pretinning Leads

• Working with pretinned leads, especially stranded wire

• Easier to solder if already tinned

Wiring a Terminal

Soldering a Terminal

Printed Circuit Boards

• Traces– Copper surface is etched away

leaving paths of copper equivalent to wired connections

• Lands– Places where leads of mounted

components come through the board– Holes that are drilled or punched

• Pads– Circles of copper around the lands

for soldering component leads

Printed Circuit Boards

Printed Circuit Boards

• Very little solder is needed for printed circuit board connectors

Switch Terminals

• Do not allow solder to run down onto the switch contact itself– Contacts are usually silver plated

• Solder to the terminals of a switch with the terminals hanging down– Gravity will keep the solder out of the

contacts– If this is not possible, use solder

sparingly and make the connection quickly, but with enough heat for good wetting action

Soldering to a Chassis

• Do not solder to aluminum chassis

• Cadmium plated steel chassis– Scrape away cadmium plating with a

knife or screwdriver– Pretin both the chassis and the braid– Use a 100 watt iron because the

chassis metal carries the applied heat away so fast, and it must be replaced quickly

Semiconductors

• Excessive heat can destroy diodes and transistors, as well as IC’s– Developed internally by exceeding

their ratings– Applied externally by soldering

• Use a heat sink– Clamp type– Copper alligator clip– Long-nose pliers with a rubber band

Wire Splices

• Splices should be avoided if possible– Should be enough terminal tie-points

on the chassis to accommodate all components by their own leads

• If splicing is necessary– Use heat shrink tubing to insulate

the wire splice to prevent shorts to other bare metal

Wire Splices

• Clean wires with emery cloth

• Twist wires together tightly

• Move hot iron along splice, as solder melts

• Solder will tend to go towards the heat

Inspection

• When you are finished, always inspect your own work for– Cold solder connections– Bridges (icicles)– Frosty looking connections

• Use a jeweler’s loupe or magnifier

Inspection

• Cross-section of a wire hooked through a terminal hole

Tacking

• Temporary solder connection– Components soldered to terminals

without shortening the leads and without hooking them into the terminal holes and bending them around

– Easy to unsolder a resistor, for example, and replace it with one of another value

– Easier if the leads and the terminals are pre-tinned

Repairs/Replacement

Soldering Aids

Soldering Aids

• Double- ended pick– One end pointed– Other end forked (bifurcated)

Soldering Aids

• Vacuum desoldering iron– Depress the bulb– Apply the heated iron to the

connection– Release the bulb

Soldering Aids

• Special desoldering tips

Soldering Aids

• Special double-edged bar for desoldering all 16 terminals of an integrated circuit chip at one time

Safety

• Wear proper eye protection• If you leave the soldering station,

turn the soldering iron off• Precautions against burns

– Always lay the iron down in the same place between connections

– Place your iron in a stand during standby

– Avoid placing flammable objects near the hot iron

– Be aware of people near you

Safety

• Inexpensive stand guards hot iron from accidental contact

Safety

• Protect wall by using metal sheet

Classification of Burns

• Causes of burns– Radiation– Electrical– Chemical– Thermal

• Area of body– Shock is possible with 15 to 25% of

the body burned– Entire palm of the hand and front of

all fingers is about 1%

Classification of Burns

• Depth of burn– First degree

• Skin is reddened, sore, and tender, but not blistered

• Damage is to outer layer of skin only

– Second degree• Outer and adjacent layers of skin• Skin will appear red and swollen and

there will be blisters

– Third degree• Skin tissues and underlying flesh• Nerve damage

Treatment of Burns

• First aid– Reduce pain– Prevent infection

• Light burns– Cover with clean cloth or bandage– May use ointment or baking soda

• Heavy burns– Cover with a clean cloth and see a

doctor– Do not open blisters– Treat for shock

Summary

• Soldering is a chemical process that uses heat to alloy two other metals together with it

• Solder wire has a flux core that cleans oxidation from the metals

• Temperature and amount of time are crucial to the operation

• Accessories make soldering and repairs easier

• Always practice safe soldering

Review

• Methods for joining metals– Soldering– Brazing– Welding

• Electrical soldering– Joining of metals by heat for the

purpose of producing a continuous and permanent path for the flow of electricity

– Chemical process called, alloying

Review

• Solder– Combination of tin and lead in

varying proportions

• Flux– Used for cleaning surfaces prior to

making the solder joint– Can be brushed on or contained in

the core of the solder wire

Proper Soldering

• Steps– Apply heat to the connection– Apply solder– Remove solder– Remove iron

• Solder joint– Shiny (silver, not grey)– Smooth surface (not cracked)– Good wetting (flowing edges)– No unintentional bridging

Proper Soldering

• Controlled temperature– Too high

• Flux chars• Components damaged

– Too low• Solder does not flow; poor joint• Components damaged

Burns

Abc’s of Electrical Soldering, by Louis M. Dezettel

Published by Howard W. Sams & Co., 1973.