PCF8574TS datasheet(20/24 Pages) PHILIPS

20 / 24 page

2002 Nov 22

Philips Semiconductors

Product speciﬁcation

Remote 8-bit I/O expander for I2C-bus

PCF8574

13 SOLDERING

13.1

Introduction

This text gives a very brief insight to a complex technology.

A more in-depth account of soldering ICs can be found in

our

“Data Handbook IC26; Integrated Circuit Packages”

(document order number 9398 652 90011).

There is no soldering method that is ideal for all IC

packages. Wave soldering is often preferred when

through-hole and surface mount components are mixed on

one printed-circuit board. Wave soldering can still be used

for certain surface mount ICs, but it is not suitable for fine

pitch SMDs. In these situations reflow soldering is

recommended.

13.2

Through-hole mount packages

13.2.1

SOLDERING BY DIPPING OR BY SOLDER WAVE

The maximum permissible temperature of the solder is

260

°C; solder at this temperature must not be in contact

with the joints for more than 5 seconds. The total contact

time of successive solder waves must not exceed

5 seconds.

The device may be mounted up to the seating plane, but

the temperature of the plastic body must not exceed the

specified maximum storage temperature (Tstg(max)). If the

printed-circuit board has been pre-heated, forced cooling

may be necessary immediately after soldering to keep the

temperature within the permissible limit.

13.2.2

MANUAL SOLDERING

Apply the soldering iron (24 V or less) to the lead(s) of the

package, either below the seating plane or not more than

2 mm above it. If the temperature of the soldering iron bit

is less than 300

°C it may remain in contact for up to

10 seconds. If the bit temperature is between

300 and 400

°C, contact may be up to 5 seconds.

13.3

Surface mount packages

13.3.1

REFLOW SOLDERING

Reflow soldering requires solder paste (a suspension of

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

to the printed-circuit board by screen printing, stencilling or

pressure-syringe dispensing before package placement.

Several methods exist for reflowing; for example,

convection or convection/infrared heating in a conveyor

type oven. Throughput times (preheating, soldering and

cooling) vary between 100 and 200 seconds depending

on heating method.

Typical reflow peak temperatures range from

215 to 250

°C. The top-surface temperature of the

packages should preferable be kept below 220

°C for

thick/large packages, and below 235

°C for small/thin

packages.

13.3.2

WAVE SOLDERING

Conventional single wave soldering is not recommended

for surface mount devices (SMDs) or printed-circuit boards

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:

• Use a double-wave soldering method comprising a

turbulent wave with high upward pressure followed by a

smooth laminar wave.

• For packages with leads on two sides and a pitch (e):

– larger than or equal to 1.27 mm, the footprint

longitudinal axis is preferred to be parallel to the

transport direction of the printed-circuit board;

– smaller than 1.27 mm, the footprint longitudinal axis

must be parallel to the transport direction of the

printed-circuit board.

The footprint must incorporate solder thieves at the

downstream end.

• For packages with leads on four sides, the footprint must

be placed at a 45

° angle to the transport direction of the

printed-circuit board. 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 4 seconds at 250

°C.

A mildly-activated flux will eliminate the need for removal

of corrosive residues in most applications.

13.3.3

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 2 to 5 seconds

between 270 and 320

°C.

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