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10

Soldering

Introduction to Soldering Surface Mount Packages

This text gives a very brief insight to a complex

technology. A more in−depth account of soldering ICs can

be found in the ON Semiconductor “Data Handbook IC26;

Integrated Circuit Packages” (document order number 9398

652 90011). There is no soldering method that is ideal for all

surface mount IC packages. Wave soldering is not always

suitable for surface mount ICs, or for printed−circuit boards

(PCB) with high population densities. In these situations

re−flow soldering is often used.

Re−flow Soldering

Reflow soldering requires solder paste (a suspension of

fine solder particles, flux and binding agent) to be applied to

the PCB by screen printing, stencilling or pressure−syringe

dispensing before package placement. Several methods

exist for re−flowing; for example, infrared/convection

heating in a conveyor type oven.

Throughput times (preheating, soldering and cooling)

vary between 100 and 200 seconds depending on heating

method. Typical re−flow peak temperatures range from 215

to 250

°C. The top−surface temperature of the packages

should preferably be kept below 230

°C.

Wave Soldering

Conventional single wave soldering is not recommended

for surface mount devices (SMDs) or PCBs with a high

component density, as solder bridging and non−wetting can

present major problems. To overcome these problems the

double−wave soldering method was specifically developed.

If wave soldering is used the following conditions must be

observed for optimal results:

turbulent wave with high upward pressure followed by

a smooth laminar wave.

1. Larger than or equal to 1.27 mm, the footprint

longitudinal axis is preferred to be parallel to the

transport direction of the PCB;

2. Smaller than 1.27 mm, the footprint longitudinal

axis must be parallel to the transport direction of

the PCB. The footprint must incorporate solder

thieves at the downstream end.

must be placed at a 45

° angle to the transport direction

of the PCB. The footprint must incorporate solder

thieves downstream and at the side corners.

During placement and before soldering, the package must

be fixed with a droplet of adhesive. The adhesive can be

applied by screen printing, pin transfer or syringe

dispensing. The package can be soldered after the adhesive

is cured. Typical dwell time is four seconds at 250

°C. A

mildly−activated flux will eliminate the need for removal of

corrosive residues in most applications.

Manual Soldering

Fix the component by first soldering two diagonally−

opposite end leads. Use a low voltage (24 V or less)

soldering iron applied to the flat part of the lead. Contact

time must be limited to 10 seconds at up to 300

°C.

When using a dedicated tool, all other leads can be

soldered in one operation within two to five seconds

between 270 and 320

°C.

Table 9. Soldering Process

Package

Soldering Method

Wave

Re−flow (Note 8)

BGA, SQFP

Not suitable

Suitable

HLQFP, HSQFP, HSOP, HTSSOP, SMS

Not suitable (Note 9)

Suitable

PLCC (Note 10), SO, SOJ

Suitable

Suitable

LQFP, QFP, TQFP

Not recommended (Notes 10 and 11)

Suitable

SSOP, TSSOP, VSO

Not recommended (Note 12)

Suitable

8. All surface mount (SMD) packages are moisture sensitive. Depending upon the moisture content, the maximum temperature (with respect

to time) and body size of the package, there is a risk that internal or external package cracks may occur due to vaporization of the moisture

in them (the so called popcorn effect). For details, refer to the drypack information in the “Data Handbook IC26; Integrated Circuit Packages;

Section: Packing Methods.”

9. These packages are not suitable for wave soldering as a solder joint between the PCB and heatsink (at bottom version) can not be achieved,

and as solder may stick to the heatsink (on top version).

10.If wave soldering is considered, then the package must be placed at a 45° angle to the solder wave direction. The package footprint must

incorporate solder thieves downstream and at the side corners.

11. Wave soldering is only suitable for LQFP, TQFP and QFP packages with a pitch (e) equal to or larger than 0.8 mm; it is definitely not suitable

for packages with a pitch (e) equal to or smaller than 0.65 mm.

12.Wave soldering is only suitable for SSOP and TSSOP packages with a pitch (e) equal to or larger than 0.65 mm; it is definitely not suitable

for packages with a pitch (e) equal to or smaller than 0.5 mm.

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