Philips Semiconductors

PIP202-12M

DC to DC converter powertrain

Product data

Rev. 01 — 15 July 2002

15 of 20

9397 750 10031

© Koninklijke Philips Electronics N.V. 2002. All rights reserved.

15. Soldering

15.1 Introduction to soldering MLF packages

The MicroLeadFrame package (MLF) is a near Chip Scale Package (CSP) with a

copper leadframe. It is a leadless package, where electrical contact to the printed

circuit board is made through metal pads on the underside of the package. In addition

to the small pads around the periphery of the package, there are large pads on the

underside that provide low thermal resistance, low electrical resistance, low

inductance connections between the power components inside the MLF package and

the PCB. It is this feature of the MLF package that makes it ideally suited for VRM

applications.

Electrical connection between the package and the printed circuit board is made by

printing solder paste on the printed circuit board, placing the component and

reflowing the solder in a convection or infra-red oven. The solder reflow process is

shown in Figure 18 and the typical temperature profile is shown in Figure 19. To

packages such as MLF, and the time above liquidus temperature should be less than

1.25 minutes. The maximum temperature can be increased for lead free solder. The

ramp rate during preheat should not exceed 3 K/s. Nitrogen purge is recommended

during reflow.

Fig 18. Typical reflow soldering process flow.

Fig 19. Typical reflow soldering temperature profile.

SOLDER PASTE

PRINTING

POST PRINT

INSPECTION

COMPONENT

PLACEMENT

PRE REFLOW

INSPECTION

REFLOW SOLDERING

POST REFLOW INSPECTION

(PREFERABLY X-RAY)

REWORK AND

TOUCH UP

03aj25

03aj26

0

100

200

300

01

23

time (minutes)

Temp

Tr

Te

Tp

rate of rise of

temperature < 3 K/s

1 min max

1.25 min max

(

°C)