NXP Semiconductors AN10365, Примечание к применению

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AN10365_3 © NXP B.V. 2008. All rights reserved.
Application note Rev. 03 — 22 April 2008 3 of 24
NXP Semiconductors
AN10365
Surface mount reflow soldering description
1. Introduction
This application note provides guidelines for board mounting of surface mount IC
packages. Nowadays, reflow soldering is a widely spread technology for soldering of
surface mount IC packages. For some of the newer IC packages, such as Ball Grid Arrays
(BGAs), reflow soldering is the only suitable method.
This application note describes the materials for reflow soldering: the Printed-Circuit
Board (PCB), IC packages and solder paste. One of the key features of the PCB is the
footprint design. The footprint design describes the recommended solder land on the PCB
to make a reliable solder joint between the IC package and the PCB. A proven solder
material is SnPb, but due to legislation, the industry has changed, to a large extent, to
Pb-free solutions such as SAC. Process requirements for solder paste printing and reflow
soldering, for SnPb and Pb-free, are also discussed in this application note. This
document concludes with a section about inspection and repair.
2. Materials
2.1 Printed-circuit boards and footprints
Printed-Circuit Boards (PCBs) are not only used as mechanical carriers for electronic
components; they also provide the electronic interconnection between these components
and also between these components and the outside world. These electronic components
may be ICs, or other types such as capacitors and resistors. Through component
selection and the use of Cu interconnections between the components, an electronic
system, such as a mobile phone, can be assembled on a PCB. The substrates used for
mounting the packages can be made of a variety of materials, including FR4 and flexible
polymers.
Due to the increased transistor density in the latest IC technologies, generation of heat
has become a major limitation of IC performance. By applying an exposed pad or heat
sink in the IC package, in combination with thermal vias in the PCB, the heat can be
transferred from the active die to the outside world. Four examples of vias capped in
different ways, are shown in
Figure 1 . Note that the only difference lies in the solder resist
pattern.
Common board finishes include NiAu, Organic Solderability Preservative (OSP), and
immersion Sn. Although finishes may look different after reflow, and some appear to have
better wetting characteristics than others, all common finishes can be used, provided that
they are in accordance with the specifications.
Fig 1. Capped vias; from left to right: via tenting from top, via tenting from bottom, via
capping from bottom, via encroached from bottom
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