Surface mount packages

AN533

20/22

Figure 19.

ST ECOPACK recommended soldering reflow profile for small packages

For each individual board, the appropriate heat profile has to be adjusted experimentally.

The current proposal is just a starting point. In every case, the following precautions have to

be considered:

●

Always preheat the device. The purpose of this step is to minimize the rate of

temperature rise to less than 3 °C per second (recommended 2 °C/s) to minimize the

thermal shock on the component.

●

Dryout section, after preheating, ensures that the solder paste is fully dried before

starting reflow step. Also, this step allows the temperature gradient on the board to be

evened out.

●

Peak temperature should be at least 30 °C higher than the melting point of the solder

alloy chosen to ensure the reflow quality. In any case the peak temperature should not

exceed 260 °C.

Lead-free devices are described in an internal specification defining:

●

their characteristics: lead-free connection coating, solderability and identification

features

●

their reliability such as soldering resistance, reliability performances, whiskers risk

prevention.

This specification is available for ST customers upon request (Title: ECOPACK Components

Definition & Characteristics). Please consult that document for further information about

reflow and wave soldering.

Voids pose a difficult reliability problem for large surface mount devices. Such voids under

the package result in poor thermal contact and the high thermal resistance leads to

component failures.

Coplanarity between the substrate and the package can be easily verified. The quality of the

solder joints is very important for two reasons:

●

Poor quality solder joints directly result in poor reliability.

●

Solder thickness affects the thermal resistance significantly. Thus, tight control of this

parameter results in thermally efficient and reliable solder joints.

0

01

2345

67

Time (min)

Temperature (°C)

2°C/s recommended

6°C/s max

220°C

125 °C

260°C max

255°C

180°C

90 sec max

10-30 sec

90 to 150 sec

3°C/s max

0

01

2345

67

Time (min)

Temperature (°C)

2°C/s recommended

6°C/s max

220°C

125 °C

260°C max

255°C

180°C

90 sec max

10-30 sec

90 to 150 sec

3°C/s max