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AN1005 - 1

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5AN1005

Surface Mount Soldering Recommendations

Introduction

The most important consideration in reliability is achieving a good

solder bond between surface mount device (SMD) and substrate

since the solder provides the thermal path from the chip. A good

bond is less subject to thermal fatiguing and will result in

improved device reliability.

The most economic method of soldering is a process in which all

different components are soldered simultaneously, such as

DO-214, Compak, TO-252 devices, capacitors, and resistors.

Reflow Of Soldering

The preferred technique for mounting microminiature compo-

nents on hybrid thick- and thin-film is reflow soldering.

The DO-214 is designed to be mounted directly to or on thick-film

metallization which has been screened and fired on a substrate.

The recommended substrates are Alumina or P.C. Board mate-

rial.

Recommended metallization is silver palladium or molymanga-

nese (plated with nickel or other elements to enhance solderabil-

ity). For more information, consult Du Pont's Thick-Film

handbook or the factory.

It is best to prepare the substrate by either dipping it in a solder

bath or by screen printing a solder paste.

After the substrate is prepared, devices are put in place with

vacuum pencils. The device may be laid in place without special

alignment procedures since it is self-aligning during the solder

reflow process and will be held in place by surface tension.

For reliable connections, keep the following in mind:

(1) Maximum temperature of the leads or tab during the solder-

ing cycle does not exceed 275 °C.

(2) Flux must affect neither components nor connectors.

(3) Residue of the flux must be easy to remove.

Good flux or solder paste with these properties is available on the

market. A recommended flux is Alpha 5003 diluted with benzyl

alcohol. Dilution used will vary with application and must be

determined empirically.

Having first been fluxed, all components are positioned on the

substrate. The slight adhesive force of the flux is sufficient to

keep the components in place.

Because solder paste contains a flux, it has good inherent adhe-

sive properties which eases positioning of the components. Allow

flux to dry at room temperature or in a 70 °C oven. Flux should be

dry to the touch. Time required will depend on flux used.

With the components in position, the substrate is heated to a

point where the solder begins to flow. This can be done on a

heating plate, on a conveyor belt running through an infrared tun-

nel, or by using vapor phase soldering.

In the vapor phase soldering process, the entire PC board is uni-

formly heated within a vapor phase zone at a temperature of

approximately 215 °C. The saturated vapor phase zone is

obtained by heating an inert (inactive) fluid to the boiling point.

The vapor phase is locked in place by a secondary vapor. (Figure

AN1005.1) Vapor phase soldering provides uniform heating and

prevents overheating.

Figure AN1005.1

Principle of Vapor Phase Soldering

No matter which method of heating is used, the maximum

allowed temperature of the plastic body must not exceed 250 °C

during the soldering process. For additional information on tem-

perature behavior during the soldering process, see Figure

AN1005.2 and Figure AN1005.3.

Figure AN1005.2

Reflow Soldering Profile

Transport

Cooling pipes

PC board

Heating

elements

Boiling liquid (primary medium)

Vapor phase

zone

Vapor lock

(secondary

medium)

Time (Seconds)

100

120

140

160

180

200

220

240

120

150

180

210

240

270

300

Pre-heat

Soak

Reflow

Cool

Down

0.5 - 0.6 ˚C/s

1.3 - 1.6 ˚C/s

<2.5 ˚C/s

Peak Temperature

220 ˚C - 245 ˚C

Soaking Zone

Reflow Zone

Pre-heating Zone

( 2 min. MAX )

60 - 90 s typical

( 2-4 min MAX )

( 2 min. MAX )

30 - 60 s typical

260

AN1005

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