Soldering Surface
Mount Components

Proper soldering maximizes the strength and conductivity of the connection
Introduction
Soldering is the process of using a metal alloy with a low
melting temperature (solder) to fuse the electrical contactsof a component to the pads of a circuit board. Proper
soldering maximizes the strength and conductivity of theconnection. Poor soldering can result in weak connections,higher resistance that causes heat buildup at the connection, and possible failure of the component.
The type of components and the pads to which they will
be attached dictate the appropriate soldering method.
The correct amount and duration of heat to be applied
is a function of the heat transfer characteristics of thecomponent, the circuit board, the solder pads, the solder(and flux), and the environment in which the soldering
takes place. For this reason, effective soldering requires
reasonably controlled conditions. Some experimentation
is usually required to determine the optimal conditions foreach application.

Types of Solder
Solder comes in the form of paste, wire (also called stringsolder), bar, and in preformed shapes. There are many
alloy combinations of solder, but the most common combination for electronics has been 63Sn/37Pb (63% Tin, 37%Lead) solder, which has a melting point of 183°C (361°F).
Environmental regulation agencies are urging the use of
low-lead or no-lead solders. A common no-lead solder
composition is 96.5% Tin / 3.5% Silver. Tin / Silver solder
has a higher melting temperature (221°C / 430°F), which
could cause process difficulties when used as a replacement for lead-based solder. The entire circuit must be
designed to withstand the higher temperatures used forthe low-lead or no-lead solders. Another environmentfriendly option, “no-clean” solders are used in applications
where post-reflow cleaning using CFC-based cleaners isto be avoided.

Flux
Soldering flux is applied to the connection area to remove
oxidation from the metal surfaces to be soldered, and to
aid in solder flow. If metal oxidation is not removed, solder
does not adhere well to the solder pad. Flux can be applied

separately, just before applying the solder, or simultaneously when using solder paste or rosincore solder.
Solder paste typically contains flux, and rosin-core solder
has a center core of flux. As these types of solder melt,
flux is released, eliminating the need for a separate flux
application.

General Soldering Guidelines
All soldering applications require the following considerations:• Preparation – Clean connections are essential tosoldering. Clean connections maximize the ability ofthe solder to adhere uniformly to the joint surfaces(wetting).
• Soldering Method – The component type and size,
and your specific application determine the soldering method.
• Materials Selection – The component contacts,
circuit board pads, solder, and flux materials mustall be compatible with the soldering method.
• Maximum Temperature – The soldering materials
and method determine the temperature profile. Allcomponents must be able to withstand the maximum exposure temperature of the soldering operation for the specified time duration.

Soldering Methods
The appropriate soldering method depends on the specificapplication, production volume and time requirements,
and the types of components to be soldered. Automatic
methods include wave, infrared (IR) reflow, air convection
reflow, radiant heating reflow, and vapor phase soldering.
Manual methods include using a soldering iron with wiretype solder, or reflow using wire solder or paste and a hot
air pencil or gun.
When using only surface mount technology (SMT) components, a reflow process is best. For thru-hole components,
and where thru-hole and SMT components are combinedon the same board, a wave soldering process is typicallyused. In some cases, combinations of wave soldering andreflow soldering are used. When manually soldering SMT
components, a nickel barrier is strongly recommended.
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Thermal Profiles
With any soldering method, the optimal preheating, heating, and cooling cycle chosen for a particular method
results in complete wetting of all solder joints and minimized thermal stresses. Too little heat does not allow thesolder to melt and flow properly. Too much heat can cause
oxidation to take place before the solder solidifies, resulting in a “cold” solder connection. Too much heat can also
cause damage to the component or circuit board. Coldsolder connections are weaker and have higher resistancethan a normal connection. Heating or cooling too quickly
can cause thermal shock leading to cracked materials. The
recommended cooling method is always gradual coolingto room temperature in order to avoid thermal shock.

Chip Inductor

Power Inductor

Reflow Profile
As with all soldering methods, the optimal reflow profile
for a circuit board assembly is dependent on the solder
material, solder amount, flux, temperature limit of eachsoldered component, heat transfer characteristics of thecircuit board and component materials, and the layoutof all components. For example, a component with aheat-sink element attached may require a longer heating
cycle than other components on the same circuit board.
The temperature versus time limitation of the least robust
component of the circuit board assembly ultimately may
determine the actual temperature profile that must be used.
For these reasons, Coilcraft does not specify solderingprofiles for our components.
Contact the solder, circuit board and component manufacturers to determine the temperature limits for yourparticular application. See Temperature Limits for our chipand power inductor maximum temperature specifications.

Solder AmountAn optimal solder fillet is determined by part size, termination style, and pad geometry. Experimentation isrecommended to establish the amount of solder to beapplied to the connection. The solder fillet radius should
be approximately the height of the inductor terminal. Optimal solder fillets for a chip inductor, power inductor, and“Spring” inductor are shown in Figure 1.

Part Positioning
Tombstoning (also called drawbridging) is when one endof a component tips up during soldering. It results fromuneven forces on the ends of the part. To avoid tombstoningor other movement of SMT components during soldering,ensure that the solder quantity, positioning, and heating
is even at all solder joints.
The magnetic fields of unshielded inductors couple whenplaced in close proximity to each other. The closer thespacing, the stronger the magnetic coupling. To minimize

Spring Inductor

Figure 1. Optimal Solder Fillets
inductor coupling, avoid narrow spacing between solder
lands of adjacent inductors.

Adhesives
If an adhesive is used to pre-attach the component, makesure that it is kept as far as possible from the solder joint
area. If too much adhesive is used, it can creep into thesolder joint, causing soldering problems.

Reworking Soldered Joints
When manually reworking soldered joints, hot air reflow isrecommended as a more controlled method than using asoldering iron. If a soldering iron must be used, follow the
manual soldering technique described later.

Chip Inductor Soldering
Use the following table as a guideline for selecting asoldering method based on the size of a surface-mountcomponent. The preference of a soldering method is basedon temperature control and even heat distribution. Reflow
is always the preferred automatic method for even andcontrolled soldering. Wave soldering is discouraged as it
requires the part to be secured to the board in a separate
operation before soldering. When soldering manually,a hot air pencil provides a reasonably controlled andevenly distributed amount of heat to a solder connection
without making direct contact. Soldering smaller size
chip inductors with a soldering iron is discouraged due tolack of temperature control, although it may be possible
under carefully controlled conditions. Only chip inductorswith a nickel barrier as part of the termination should besoldered manually.
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Soldering Method
AutomaticManual

Size

Reflow

Wave

Hot Air
PencilSoldering
Iron

0201

PA

P

A

0302

P

A

P

A

0402

PA

P

A

0603

P

A

P

O

0805

P

A

P

O

1008

PA

P

O

1206

P

A

PO

1812

P

A

P

O

Key: P = Preferred; O = Optional; A = Avoid

Temperature Limits
Our chip inductors meet the following resistance to soldering heat condition: Three reflows at >217°C for 90 seconds(+260°C ±5°C for 20–40 seconds), allowing parts to cool
to room temperature between.

Power Inductor Soldering
The same logic for choosing a chip inductor soldering
method can be applied to power inductor soldering. Reflow is the preferred method for SMT components. Other
methods are possible, but typically do not offer processcontrols comparable to reflow methods.

Temperature Limits
Our power inductors meet the following resistance tosoldering heat condition: Three reflows at >217°C for 90
seconds (+260°C ±5°C for 20–40 seconds), allowing parts
to cool to room temperature between.

Manual (Hand) Soldering Technique
While the amount of solder, and the amount and durationof heat to be applied are application-specific, the following
general hand-soldering guidelines will lead to consistentand reliable solder connections. A hot air pencil is preferred
for even heat application and control, however, the following technique applies to hand-soldering of surface-mountor thru-hole electronic components using rosin-core solder
and a soldering iron.

Preparation
Before beginning the soldering process, identify the solder
composition and flux type (if you’re not using rosin core solder). The solder type dictates the appropriate temperatureof the soldering iron tip. Use small diameter wire solder
for soldering small SMT components.
Before heating the soldering iron, select an appropriate
size tip. Use a smaller tip for fine work and a larger tip forlarger connections. Clean the tip of any contamination oroxidation. Try not to scratch the tip surface, as scratchescause the tip to lose heat too quickly. Place a sponge,
soaked in cold water, nearby for frequent tip cleaning
between soldering operations.
Clean the electronic component’s contacts/leads and thecircuit board pads of any contamination or residue.

Soldering Iron Temperature
The optimum temperature setting is the lowest one thatallows the solder to melt completely and flow into thesolder connection.
A temperature-controlled soldering iron is recommended,
especially for fine wire applications. Check the solder
manufacturer’s temperature recommendations for thespecific solder type. The solder manufacturer may only
provide the melting temperature range, so you may haveto experiment to determine the appropriate soldering irontip temperature.

Place the Component
Center the electronic component on the circuit board
mounting pads. For thru-hole components, feed the leads
through the holes and bend the end of the lead over tohold it in place.

Tin Soldering Iron Tip
Apply a small amount of solder to the heated soldering
iron tip. This aids in conducting heat to the solder connection and prevents the tip from overheating. A clean,tinned soldering iron tip appears shiny. An overheated orburnt tip appears brown or black. Water soluble flux tends
to wick up and away from the soldering iron tip, causing itto overheat. No-clean solders contain less flux, and may
require more frequent tinning of the tip.
Pre-tinning the component leads/contacts and the circuitboard pads can also improve the solderability of the connection.

Heat the Connection
If you are using a separate flux, apply a small amount tothe connection area.
Apply the soldering iron tip to the opposite side of the connection to where the solder is applied. Hold the tip of thesoldering iron for a few seconds against the connection
so that it touches the component lead/contact and thecircuit board pad. For larger connections, more heatingtime may be required.

Apply the Solder
Touch the end of the solder wire to the heated connection
on the side opposite of the soldering iron tip. Allow the solder to flow into the connection. If the heating is sufficient,
the solder spreads out into the connection and wicks up

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onto the component lead/contact. Once this happens,stop adding solder, and then remove the soldering iron.

Further reference

Do not move the component or board until the solder
connection cools for a few seconds. Moving the component or board before it cools can result in a “cold” solderconnection.

PCB Washing and Coilcraft Parts

When soldering is complete, clip off excess lead lengths
close to the connection. Use an ohmmeter to check for ashort or open connection.

Selecting Flux for Soldering Coilcraft Components
Kester, Inc. (www.kester.com)
Indium Corporation of America (www.indium.com)
Alpha Metals (www.alphametals.com)

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