Vicor Micro Family of DC-DC Converter - Manual (Page 63)

Design Guide & Applications Manual
For Maxi, Mini, Micro Family DC-DC Converters and Configurable Power Supplies
Maxi, Mini, Micro Design Guide Rev 4.9 vicorpower.com
Page 62 of 88
Apps. Eng. 800 927.9474 800 735.6200
13. Recommended Soldering Methods, Lead Free Pins (RoHS)
The following chapters contain soldering information for
the following Vicor product families; Maxi, Mini, Micro;
VE-200, VE-J00; VI BRICK
® , and similar package filters and
front-ends. This document is intended to provide guidance
for making high-quality solder connections of RoHS-
compliant Vicor power modules to printed circuit boards.
This application note applies to lead-free soldering of
Vicor’s RoHS- compliant modules. The following provides
an outline for appropriate soldering procedures and the
evaluation of solder joints to ensure an optimal connec-
tion to the power module. Common soldering defects will
be examined and direction will be provided for detecting
and handling them. Vicor’s manufacturing facilities use the
IPC-A-610 standards for establishing quality solder joints.
It is recommended that manufacturing processes using
Vicor modules refer to these same standards, which can
be found, along with supporting documentation, at
www.ipc.org .
ANALYSIS OF A GOOD SOLDER JOINT
The IPC-A-610 standard requires that solder fill at least
75% of the barrel to ensure a solid connection. Ideally, all
connections should have a 100% fill. To accomplish this,
the solder applied to both the barrel and the pin must
exhibit a process known as wetting. Wetting occurs when
liquid solder on a surface is heated to the point that it
loses a significant amount of latent surface tension and
evenly coats the surface via capillary action (both cohesion
and adhesion).
During the soldering process wetting can be identified by
an even coating of solder on the barrel and pin. In
addition, coating the surface of barrel and pin, the solder
will gather at the intersection of the two and produce a
trailing fillet along each surface. Once wetting has occurred,
then upon solidification it will bond appropriately to both
components, producing a quality connection.
Figure 13–1 shows a side profile of a good solder joint
with a power module. Notice that the solder forms a
concave meniscus between pin and barrel. This is an
example of a properly formed fillet and is evidence of
good wetting during the soldering process. The joint
between solder and pin as well as solder and pad should
always exhibit a feathered edge. In Figure 13–1 it can also
be seen that the solder covers a good deal of the surface
area of both the pin and the pad. This is also evidence of
good wetting. (Notice also that the solder joint is dull
compared to leaded processing). This is evidence of good
immobilization of the joint during cooling as well as good
cleaning of the board prior to soldering. All soldering con-
nections should exhibit similar characteristics regardless of
whether they are soldered by hand, by fountain, or by
wave.
In examining a solder joint, be sure that there is no solder
connecting one pad to another. This is known as a solder
bridge and will be discussed later.
SOLDERING PROCEDURES
Hand Soldering. Before soldering, make sure that the
PCB is clean and free of debris, chemical residue, or liquid.
It is not recommended that additional flux other than
what is contained in the solder be used during soldering
because it potentially leaves a residue that cannot be
removed without potentially damaging or compromising
the power module. Also, the presence of these residues
on the modules may cause harm or improper operation.
The pins on Vicor modules are optimized to provide a low-
resistance electrical connection. The final mounting
scheme for any module should be designed to minimize
any potential mechanical stress on the pins and solder
joints. Modules with heat sinks or modules used in
systems that are subject to shock or vibration should use
standoffs to minimize stress on the pins. It is not recom-
mended that discrete wires or connectors be soldered
directly onto a module.
Also necessary for a good solder connection is pin protrusion
from the PCB. It is not possible to create a good solder
joint without some protrusion of module pins from the
PCB. If the PCB is too thick to allow good pin protrusion,
consider using Vicor module accessories such as sockets to
allow proper mounting. Before soldering, the module
should be mechanically affixed or immobilized with
Figure 13–1 — Side profile of Maxi or Mini module’s RoHS solder joint.
OVERVIEW