Soldering Surface Mount Components

Document 362-1

Specifications subject to change without notice. Document 362-1 Revised 10/09/03

IntroductionSoldering is the process of using a metal alloy with a lowmelting temperature (solder) to fuse the electrical con-tacts of a component to the pads of a circuit board. Propersoldering maximizes the strength and conductivity of theconnection. Poor soldering can result in weak connec-tions, higher resistance that causes heat buildup at theconnection, and possible failure of the component.

The type of components and the pads to which they will beattached dictate the appropriate soldering method. Thecorrect amount and duration of heat to be applied is afunction of the heat transfer characteristics of the compo-nent, the circuit board, the solder pads, the solder (andflux), and the environment in which the soldering takesplace. For this reason, effective soldering requires reason-ably controlled conditions. Some experimentation is usu-ally required to determine the optimal conditions for eachapplication.

Types of SolderSolder comes in the form of paste, wire (also called stringsolder), bar, and in preformed shapes. There are manyalloy combinations of solder, but the most commoncombination for electronics has been 63Sn/37Pb(63% Tin, 37% Lead) solder, which has a melting point of183°C (361°F).

Environmental regulation agencies are urging the use oflow-lead or no-lead solders. A common no-lead soldercomposition is 96.5% Tin / 3.5% Silver. Tin / Silver solderhas a higher melting temperature (221°C / 430°F), whichcould cause process difficulties when used as a replace-ment for lead-based solder. The entire circuit must be de-signed to withstand the higher temperatures used for thelow-lead or no-lead solders. Another environment-friendlyoption, “no-clean” solders are used in applications wherepost-reflow cleaning using CFC-based cleaners is to beavoided.

FluxSoldering flux is applied to the connection area to removeoxidation from the metal surfaces to be soldered, and toaid in solder flow. If metal oxidation is not removed, solderdoes not adhere well to the solder pad. Flux can be appliedseparately, just before applying the solder, or simultaneouslywhen using solder paste or rosin-core solder.

Solder paste typically contains flux, and rosin-core sol-der has a center core of flux. As these types of solder

melt, flux is released, eliminating the need for a separateflux application.

We recommend avoiding water-soluble fluxes and clean-ers, which may lead to corrosion of exposed coppersurfaces.

General Soldering GuidelinesAll soldering applications require the following consider-ations:

• Preparation – Clean connections are essential to sol-dering. Clean connections maximize the ability of thesolder to adhere uniformly to the joint surfaces (wetting).

• Soldering Method – The component type and size,and your specific application determine the solderingmethod.

• Materials Selection - The component contacts, circuitboard pads, solder, and flux materials must all be com-patible with the soldering method.

• Maximum Temperature – The soldering materials andmethod determine the temperature profile. All compo-nents must be able to withstand the maximum expo-sure temperature of the soldering operation for thespecified time duration.

Soldering MethodsThe appropriate soldering method depends on the spe-cific application, production volume and time requirements,and the types of components to be soldered. Automaticmethods include wave, infrared (IR) reflow, air convectionreflow, radiant heating reflow, and vapor phase soldering.Manual methods include using a soldering iron with wire-type solder, or reflow using wire solder or paste and a hotair pencil or gun.

When using only surface mount technology (SMT) com-ponents, a reflow process is best. For thru-hole compo-nents, and where thru-hole and SMT components arecombined on the same board, a wave soldering process istypically used. In some cases, combinations of wave sol-dering and reflow soldering are used.

Thermal ProfilesWith any soldering method, the optimal preheating, heat-ing, and cooling cycle chosen for a particular method re-sults in complete wetting of all solder joints and minimizedthermal stresses. Too little heat does not allow the solderto melt and flow properly. Too much heat can cause oxida-tion to take place before the solder solidifies, resulting in a

Document 362-2

Specifications subject to change without notice. Document 362-2 Revised 10/09/03

“cold” solder connection. Too much heat can also causedamage to the component or circuit board. Cold solderconnections are weaker and have higher resistance thana normal connection. Heating or cooling too quickly cancause thermal shock leading to cracked materials. Therecommended cooling method is always gradual coolingto room temperature in order to avoid thermal shock.

Reflow ProfileAs with all soldering methods, the optimal reflow profile fora circuit board assembly is dependent on the solder mate-rial, solder amount, flux, temperature limit of each sol-dered component, heat transfer characteristics of the circuitboard and component materials, and the layout of all com-ponents. For example, a component with a heat-sink ele-ment attached may require a longer heating cycle thanother components on the same circuit board. The tem-perature vs. time limitation of the least robust componentof the circuit board assembly ultimately dictates the opti-mal temperature profile.

Contact the solder, circuit board and component manufac-turers to determine the temperature limits for your particu-lar application. See Temperature Limits for our chip andpower inductor maximum temperature specifications.

Solder AmountAn optimal solder fillet is determined by part size, termina-tion style, and pad geometry. Experimentation is recom-mended to establish the amount of solder to be applied tothe connection. The solder fillet radius should be approxi-mately the height of the inductor terminal. Optimal solderfillets for a chip inductor, power inductor, and “Spring” in-ductor are shown in Figure 1.

Part PositioningTombstoning (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 iseven 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 minimizeinductor coupling, avoid narrow spacing between solderlands of adjacent inductors.

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

Reworking Soldered JointsWhen 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 themanual soldering technique described later.

Chip Inductor SolderingUse the following table as a guideline for selecting a sol-dering method based on the size of a surface-mount com-ponent.

Figure 1. Optimal Solder Fillets

Chip Inductor

Power Inductor

Spring Inductor

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

eziS

dohteMgniredloS

citamotuA launaM

wolfeR evaWriAtoHlicneP

gniredloSnorI

1020 P A P A

2030 P A P A

2040 P A P A

3060 P A P O

5080 P A P O

8001 P A P O

6021 P A P O

2181 P A P O

The preference of a soldering method is based on tem-perature control and even heat distribution. Reflow is al-ways the preferred automatic method for even andcontrolled soldering. Wave soldering is discouraged as itrequires the part to be secured to the board in a separateoperation before soldering. When soldering manually, ahot air pencil provides a reasonably controlled and evenlydistributed amount of heat to a solder connection withoutmaking direct contact. Soldering smaller size chip induc-tors with a soldering iron is discouraged due to lack oftemperature control, although it may be possible undercarefully controlled conditions.

Temperature LimitsOur chip inductors meet the requirements of the “Resis-tance to Soldering Heat” qualification standard, describedin our Chip Inductor Qualification Standards. The Resis-tance to Soldering Heat test method and specification areas follows:

Test method/conditionInductors shall be reflowed onto a PC board using 63 Sn/37 Pb or 96.5 Sn/3.5 Ag solder paste. Solder process shallbe 230°C for 20 ±2 seconds and 260°C for 5 ±2 seconds.

SpecificationThere must be no case deformation or change in dimen-sions. Inductance shall not change more than the statedtolerance.

Power Inductor SolderingThe same logic for choosing a chip inductor solderingmethod can be applied to power inductor soldering. Re-flow is the preferred method for SMT components. Othermethods are possible, but typically do not offer processcontrols comparable to reflow methods.

Temperature LimitsOur power inductors meet the requirements of the “Resis-tance to Soldering Heat” qualification standard, describedin our Power Inductor Qualification Standards. The Resis-tance to Soldering Heat test method and specification areas follows:

Test method/conditionInductors shall be reflowed onto a PC board using 63 Sn/37 Pb or 96.5 Sn/3.5 Ag solder paste. Solder process shallbe 230°C for 20 ±2 seconds and 260°C for 5 ±2 seconds.

SpecificationInductance shall not change more than the stated toler-ance.

Manual (Hand) Soldering TechniqueWhile the amount of solder, and the amount and durationof heat to be applied are application-specific, the followinggeneral hand-soldering guidelines will lead to consistentand reliable solder connections. A hot air pencil is pre-ferred for even heat application and control, however, thefollowing technique applies to hand-soldering of surface-mount or thru-hole electronic components using rosin-coresolder and a soldering iron.

PreparationBefore beginning the soldering process, identify the soldercomposition and flux type (if you’re not using rosin coresolder). The solder type dictates the appropriate tempera-ture of the soldering iron tip. Use small diameter wire sol-der for soldering small SMT components.

Before heating the soldering iron, select an appropriatesize 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 be-tween soldering operations.

Clean the electronic component’s contacts/leads and thecircuit board pads of any contamination or residue.

Soldering Iron TemperatureThe optimum temperature setting is the lowest one thatallows the solder to melt completely and flow into the sol-der connection.

A temperature-controlled soldering iron is recommended,especially for fine wire applications. For standard 63Sn/37Pb rosin core solder, set the solder tip temperature to amaximum of 280°C (536°F). If you are using no-clean 63Sn/37Pb solder, set the tip to ~260°C (500°F).

If a different type of solder is to be used, check the soldermanufacturer’s recommendations for the specific soldertype. The solder manufacturer may only provide the melt-ing temperature range, so you may have to experiment todetermine the appropriate soldering iron tip temperature.

Place The ComponentCenter the electronic component on the circuit board mount-ing pads. For thru-hole components, feed the leads throughthe holes and bend the end of the lead over to hold it inplace.

Tin Soldering Iron TipApply a small amount of solder to the heated solderingiron tip. This aids in conducting heat to the solder connec-tion and prevents the tip from overheating. A clean, tinnedsoldering iron tip appears shiny. An overheated or burnt tip

Document 362-3

Specifications subject to change without notice. Document 362-3 Revised 10/09/03

appears brown or black. Water soluble flux tends to wickup and away from the soldering iron tip, causing it to over-heat. No-clean solders contain less flux, and may requiremore frequent tinning of the tip.

Pre-tinning the component leads/contacts and the circuitboard pads can also improve the solderability of the con-nection.

Heat The ConnectionIf you are using a separate flux, apply a small amount tothe connection area.

Apply the soldering iron tip to the opposite side of theconnection to where the solder is applied. Hold the tip ofthe soldering iron for a few seconds against the connec-tion so that it touches the component lead/contact and thecircuit board pad. For larger connections, more heatingtime may be required.

Document 362-4

Specifications subject to change without notice. Document 362-4 Revised 10/09/03

Apply The SolderTouch the end of the solder wire to the heated connectionon the side opposite of the soldering iron tip. Allow thesolder to flow into the connection. If the heating is suffi-cient, the solder spreads out into the connection and wicksup onto the component lead/contact. Once this happens,stop adding solder, and then remove the soldering iron.

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

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