Soldering Process Optimization

Soldering Process Optimization
Application Tech Note
Soldering with High-Power Diode Lasers – Process Optimization
Introduction
The trend towards miniaturization in the electronics industry has led to an increase in the
population density of circuit boards across a
whole range of products—from cell phones
to under-the-hood sensors for the automotive
industry. In these circumstances wave soldering is not always possible, and this has led to
a demand for selective soldering processes.
The use of direct-diode lasers for laser soldering has, therefore significantly increased
within the last two years. The majority of systems currently employed for laser soldering
produces joints sequentially, as opposed to
conventional soldering processes, such as
wave soldering, which produces a large number of joints simultaneously. The average
power of the laser required in most cases is
relatively low. Most joint configurations require less than 10 watts to produce a single
joint. However, there are some solder reflow
applications where a number of pads are
scanned simultaneously, and these require
higher average power. Sequential soldering
puts the process well within the power range
of different types of diode laser devices. Power
density requirements (watts/cm2) are also relatively low. Essentially a relatively slow (in laser processing terms) heating process, melting typically takes place in the 100s of milliseconds. In fact, if excess power density is
employed, poor joint quality results due
to spattering.
A number of worldwide commercial manufacturers are now offering fully integrated diode laser soldering systems. These systems
come equipped with X-Y positioning stages
and/or robots to provide precise, contactless
soldering.
Optimization of
the soldering process
A previous Application Note, “Soldering with
High-Power Laser Diodes,” laid out the general approach for laser soldering using Coherent FAP™ systems. The purpose of this new
note is to expand on this method and to provide additional information, both on how the
soldering process can be optimized and how
the defects associated with non-optimized
joints can be identified.
Since the start of the electronics industry, leadcontaining solders have been used almost exclusively. The most widely used composition
has been the low-melting point (183°C)
eutectic lead-tin (63%Sn/37%Pb) composition. Lead-free soldering has been introduced
as a response to proposed legislative restrictions on the use of lead in electronics and by
marketing activities in the Far East and
Europe. However, the majority of soldering applications continue to use this or very similar
compositions.
As board densities increase and pad size decreases, the requirement for precision in the
soldering process has led to an increasing need
for the accuracy that lasers provide. This improvement in accuracy has put increasingly
demanding requirements on the positioning
system. Two approaches are currently employed: Either positioning the workpiece beneath the stationary laser spot, or manipulating the fiber-coupled laser beam output. Both
approaches require precise alignment. Typically, this has been achieved using a visible
laser beam, usually a red laser diode, as a
‘pointer.’ There are limitations to this approach,
because the red beam never entirely nor accurately delineates the real laser spot. However,
this can be achieved if a CCD camera is
employed to directly view the laser spot, and
using accessories (now available) that allow
co-axial viewing of the laser beam in real time.
Because of the greatly reduced cost of such
cameras, almost all precision soldering stations
are now equipped with CCDs. All of the cameras supplied have some sensitivity in the
infra-red range and, hence, our approach is to
align using a remotely viewed (via a TV monitor) low-power diode laser beam. This has two
benefits: First, it is far safer. Second, because
the diameter of the laser spot does not
increase in size with an increase in power, the
exact area covered by the laser spot is seen.
This is critical if very careful alignment is
required, as is the case in many microsoldering situations.
dard 44-pin chip carrier with a 2.54 mm
(1/10") pin pitch was soldered into a standard
conformal-coated FR4 printed circuit board
material. To achieve reproducible solder joints,
1.7 mm OD (0.07") solder pre-forms were
used. The pre-form was selected over solder
paste or solder wire feeding, as the use of the
pre-forms provides more reproducible results,
especially in a laboratory environment. In this
case, 96.5%/3.5% tin/silver solder pre-forms
were used, melting point 183°C, supplied by
AlphaFry Technologies. These are coated with
an RMA-type low-solids flux, making it a
‘no clean’ solder.
The samples shown here were soldered in air
using an 810 ±10 nm Coherent FAP™
System with a total average power capability
of 30 watts. The energy was delivered via an
800 µm diameter delivery fiber and a 1:1
Optical Imaging Accessory (OIA), which had
a working distance of 33 mm and produced
an 800 µm spot.
To ensure a valid optimization trial, soldering
time was fixed at 0.8 sec.: Therefore, average
power and solder composition were the primary variables studied. Average power was
changed in increments of 2 watts over the
range at which solder joints were produced.
At each setting, a total of 13 pins were soldered. Results over the optimum range of 6 to
12 watts average power are reported here. To
confirm and to expand on visual observations,
metallographic samples were prepared for
examination. Transverse diametral cross-sections of several joints from each parameter
Lead-tin Soldering –Over-heated Joints
Experimental details
A single, standard solder joint configuration
was used to demonstrate a generic experimental approach to minimizing heat input. A stan-
Figure 1. PbSn solder, 10 watts, 0.8 sec
Soldering with High-Power Diode Lasers – Process Optimization
Lead-Tin Soldering –‘Dry’ Joints
Lead-tin Soldering –Over-heated Joints
Lead-Tin Soldering –Optimized Joints
Figure 2. PbSn solder, 10 watts, 0.8 sec
Figure 3. PbSn solder, 8 watts, 0.8 sec
Figure 5. PbSn solder, 6 watts, 0.8 sec
Figure 4. PbSn solder, 8 watts, 0.8 sec
Figure 6. PbSn solder, 6 watts, 0.8 sec
setting were prepared using conventional metallurgical techniques.
Figure 1 shows how a slight excess of power
(for a fixed solder time) produces a slightly
overheated appearance to the joint, with some
degradation of the flux apparent and a lessclean appearance.
Figure 2 is a transverse section across the
diameter of the pin (0.5 mm/0.02" diameter).
The pin has been sectioned and prepared using a standard metallurgical technique. The
section is then acid-etched to reveal the internal grain structure of the joint.
Figure 2 confirms this slightly overheated
microstructure with some porosity showing.
The profile of the joint is also confirmed to be
less than ideal. Some slumping of the solder
has occurred and grain structure generally
appears coarse.
Figure 3 shows what appears from visual
examination alone to be close to optimum parameters for this particular joint configuration.
Figure 4 shows a porosity-free microstructure
with good wicking of solder down into the
board. All contacting surfaces appear wellwetted. Grain structure appears fine.
Figure 5 shows a noticeably dry joint.
Figure 6 shows a cross-section of a dry joint
(not as serious as Figure 5). Serious porosity
and lack of wetting are both visible in this
cross-section, although the profile of the
solder joint is not as poor as that shown
in Figure 5. This suggests that at this low
power, process variability increases, as no such
lack of consistency was seen at higher
average powers.
There is evidence from all these cross-sections,
that in addition to the macro-scale defects,
there are more subtle variations in the microstructure that would significantly affect the
mechanical properties of the joint. These will
be covered in a future Application Note.
Conclusions
• High-quality solder joints can be produced using Coherent FAP™ Systems by
a straightforward process optimization
technique.
• Laser power has a critical effect on solder joint quality.
• Metallography has confirmed that initial
visual observations of solder joint quality were correct.
• Small but highly repeatable changes in
laser power can produce controlled metallurgical changes in the microstructure.
Coherent, Inc.
Produced by the Laser Application Center at:
5100 Patrick Henry Drive
Santa Clara, CA 95054
Telephone: 877-434-6337 Fax: 408-764-4329
E-mail: [email protected]
Web: www.CoherentInc.com
MC-045-02-1M0302
03/2002
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