Circuits Assembly, February 2004, electronic reprint

Selective

Soldering

he trends toward miniaturization in the
electronics industry and toward automa-
tion in the telecom equipment industry

have led to the demand for new, highly control-
lable selective laser soldering technology. In the
electronics industry, modern high-density elec-
tronic and electro-optic subassemblies frequently
include delicate heat- and/or debris-sensitive
components as well as complex three-dimension-
al (3-D) circuit geometries that cannot be sol-
dered using conventional wave or hot air solder-
ing techniques. At the same time, in the telecom
industry, the need to lower costs, improve yields
and save on real estate is resulting in the automa-
tion of many labor-intensive manufacturing
operations, including soldering.

To adapt to these trends and market shifts,

some electronics and telecom equipment manu-
facturers have adopted high-power diode laser
soldering technology because it offers process
controllability, high reliability and ease of
automation. Selective laser soldering enables the
delivery of precise amounts of energy to specific
locations, even those difficult to reach, without
causing heat-related damage to surrounding
areas or components.

Consequently, it can be used with special sub-

strates, thermally sensitive high-value compo-
nents and high-temperature and low-lead sol-
ders. In addition, laser soldering is compatible
with the conventional lead-based (Sn-Pb) and
newer lead-free (Sn-Ag) solders used in electron-
ics manufacturing, as well as with the gold-com-

patible (Au-Sn) solders employed in the telecom
industry.

To take full advantage of the precision and

controllability offered by lasers, these devices
are usually mated to automated precision x-y
positioning stages or robotic arms. Further, to
provide the high reliability required for manu-
facturing environments, high-power diode
lasers using aluminum-free active area (AAA)
technology offer increased operating lifetimes.
AAA diode lasers have longer lifetimes because
facet oxidation, the primary failure mechanism
in conventional AlGaAs semiconductor materi-
als, is absent.

Industrial Soldering

Miniaturization of electronic devices used in

automotive, consumer electronics, avionics and
biomedical applications has led to high-density
microelectronics with fine-pitch leads and small
pad diameters. These packaging configurations
are often 3-D in structure and frequently include
thermally sensitive or high-value components,
such as sensors, lenses, micro-electromechanical
systems (MEMS) and central processing units
(CPUs), that cannot be soldered with wave sol-
dering. In these cases, selective laser soldering is a
viable manufacturing solution.

Diode laser soldering provides temporal and

spatial process control. This control extends to
both the location and the metallurgy of connec-
tions, resulting in optimized joints for thermally
sensitive components, special substrates, diffi-
cult-to-reach locations and fine-pitch quad flat
packs (QFPs). Furthermore, process control may
allow manufacturers to use diode lasers to rework
poorly soldered units.

Selective laser soldering typically requires less

than 10 watts of average laser power and relative-
ly low power-density (watts/cm

) to produce a

single joint. In fact, if excess power-density is
employed, spattering can cause poor joint quality.
Despite these low power requirements, laser-sol-
dering times are on the order of hundreds of mil-
liseconds per joint. This speed, coupled with the
small size, efficiency, ease-of-use, high-reliability
and the integrated analog and digital interfaces of

Non-contact

selective

soldering

with high-

power diode

lasers.

Laser Solutions for Soldering

Tony Hoult

Circuits Assembly

FEBRUARY 2004

www.circuitsassembly.com

In the telecom industry, the need

to lower costs, improve yields and

save on real estate is resulting in

the automation of many labor-inten-

sive manufacturing operations,

including soldering.

Selective

Soldering

the lasers themselves, allows diode laser soldering to be easily
automated.

Laser soldering is also replacing resistive radio frequency (RF)

soldering, which employs resistive heating elements that are
hand-positioned in the nose of some telecom packages. Auto-
mated laser soldering using x-y positioning tables and/or robot-
ic arms improves yields and lowers costs because it is more reli-
able and has a shorter cycle time than RF soldering.

Lead-Free Soldering Developments

While lead-free efforts have recently made strong headway in

Europe, Japan and the United States, Europe has led the banning
of lead. Specifically, in 2000, a draft of the European Union’s
Waste Electrical and Electronic Equipment (WEEE) Directive
proposed banning lead dumping in European landfills by 2004

Then, agreement was reached on the WEEE Directive and the
Restriction of Hazardous Substances (RHS) Directive at a Euro-
pean Summit meeting in November 2003. These directives pro-
hibit the use of hazardous materials beginning July 1, 2006.
These directives make lead free a requirement for products on
sale to European consumers after this date. Leading manufactur-
ers are expected to conform to the following timetable one year
ahead of schedule, while other manufacturers may reach these
milestones two years later.

For components these developments mean:
• some availability of lead-free components since the end of

2001

• complete lineup of components with lead-free terminations

since the end of 2003

• complete lineup of lead-free components by the end of

2004.

For assemblies these developments mean:
• manufacturing lead-free soldered assemblies began by the

end of 2002

• complete lead elimination from products by the end of

2005.

The roadmap also recommends a solder alloy composed of

tin-silver-copper (Sn-Ag-Cu, or SAC) for board assembly and
that industry leaders develop a system for labeling.

As a result of these directives, manufacturers are replacing

low melting point (183°C) lead-based solders with newer,
higher melting point (>220°C) lead-free solders, such as SAC
and tin-silver alloys. When engineers at a laser application
center compared laser soldering of the 96.5%Sn/3.5%Ag alloy
with a Sn-Pb alloy, it was found to be easier to solder! This
result is likely due to the ease with which the higher solder
temperatures can be reached. Engineers have also demonstrat-
ed laser reflow of even higher-temperature gold-tin solders
(279°C). Therefore, on the basis of melting temperature, diode
laser soldering is likely to be compatible with all solder com-
positions.

Soldering with High-Power Laser Diodes

For soldering tasks within the microelectronics industry, an

average laser power range of 2 to 80 watts is used, depending on
solder joint dimensions and the required speed. For convenience,
soldering tasks are usually categorized by size into small, medi-
um and large soldering areas.

Circuits Assembly

FEBRUARY 2004

www.circuitsassembly.com

FIGURE 1:

Severe dry joint; Sn-Ag solder,

6W. 0.8 sec.

FIGURE 2:

Dry joint; Sn-Ag solder, 8W. 0.8 sec.

FIGURE 3:

Optimized joint; Sn-Ag solder,

10W. 0.8 sec.

FIGURE 4:

Cross section of Figure 1.

FIGURE 5:

Cross section of Figure 2.

FIGURE 6:

Cross section of Figure 3.

Selective

Soldering

www.circuitsassembly.com

Circuits Assembly

FEBRUARY 2004

Small, 40 to 100 µm (0.0016 to 0.004 in.) Pads

Typical applications for soldering these small pads are in

high-density packaging. A few watts of average power are usual-
ly sufficient to solder these joints. Specific spot size and working
distance requirements can be addressed by a host of commer-
cially available optical imaging accessories (OIAs).

Medium, 100 to 500 µm (0.004 to 0.02 in.) Pads

In these cases, 25 watts delivered from an 800-µm-diameter

optical fiber with an OIA capable of reducing the source size by
a factor of 1.8:1 is preferred. Dwell time is approximately 1 sec-
ond per solder joint.

Large, 1 to 3 mm (0.04 to 0.12 in.) Pads

In some cases, scanning several solder joints simultaneously is

necessary and can be accomplished by expanding the diode laser
spot size. This technique is a viable solution for soldering multi-
ple joints on a densely populated printed circuit board (PCB). To
increase the scan time, higher average power diode laser systems
(up to 80 watts continuous wave) can be employed.

To demonstrate the high joint quality achievable with laser

soldering, in particular the quality of lead-free joints, a series of
trials was conducted. The lead-free samples shown were laser sol-
dered with a fixed time of 0.8 seconds, but at different average
power levels to determine optimum soldering parameters. The

joints were subsequently cross-sectioned using conventional
metallurgical techniques.

Figures 1 and 4 show an extreme case of dry joint caused by

under-heating; almost no wicking (capillary action) into the
through hole is visible. Figures 2 and 5 also show a poorly wetted
joint, but at a slightly higher average power. Note the porosity in
the joint and the subtle variations in the microstructure that would
adversely affect the mechanical properties of the joint. Finally, Fig-
ure 3 shows optimum laser parameters for this particular joint
configuration. Figure 6 confirms this well-controlled soldering
process with clean, non-oxidized joints. Figure 6 also shows a fine-
grain, porosity-free microstructure with good solder wicking
down into the board and well-wetted contacting surfaces.

Although the majority of these joints has been prepared

with a laser emitting at the widely used 810-nm wavelength,
new evidence has emerged that some solder mask coatings are
more damage tolerant to lasers emitting in the 940- to 980-nm
regime.

Gold Soldering for Photonics Applications

Gold-metallized telecom fibers have also been successfully

laser soldered to gold-plated substrates using diode lasers at rel-
atively low average powers. Specifically, engineers tested a stan-
dard 80/20 Au-Sn eutectic solder composition with a melting
point of 279°C. By pulsing the output of the diode laser for

Selective

Soldering

rapid, but controlled, heat input and minimizing process time,
laser soldering created high quality solder joints with large-
grained, finely divided eutectic or near-eutectic joint micro-
structures.

Diode laser soldering enables precise amounts of energy to be

delivered to specific soldering locations without causing collater-
al heat-related damage. Hence, even small-diameter, single-
mode telecom fibers, which are easily deformed by excess heat
input, can be safely soldered. Furthermore, laser soldering cre-
ates stable joints that can position the fiber with submicron
accuracy to ensure long-term performance and to maximize
optical signal transmission of single-mode fibers. Fine fiber
alignment is performed while the laser is being used to selective-
ly reflow the joint.

The telecom equipment industry is now starting to use diode

lasers to automate the fiber-soldering process in an ongoing
effort to reduce high-volume production costs associated with
manual labor and to improve product quality, consistency, yield
and throughput. In the past, the operator’s skill and experience
were critical to yield and throughput. Automated and highly
controllable diode laser soldering of gold-metallized fibers may
offer greater yields, higher precision, reduced cycle time and low-
ered costs compared to hand soldering.

Putting Laser Soldering Technology To Use

Typical diode laser soldering systems consist of a laser/control

unit and a fiber cable to allow easy delivery of laser light to any
desired location. Consequently, the laser/control unit can be
located remotely or rack mounted, while the laser light is deliv-
ered via armored optical fiber to the production line. As solid-
state devices, diode lasers offer wall-plug efficiency (>40%) using
single-phase electricity, which results in a low cost of ownership.
Furthermore, their ease of operation and integrated analog and
digital interfaces make them easily adaptable to automation in
manufacturing environments.

Widely used fiber-coupled units provide up to 30 watts of

average output power via an 800-micron-diameter fiber. The
laser/control unit provides diode laser temperature and current
control, and output can be pulsed using internal and external
system interfaces, including RS232 and IEEE-488. These systems
may have up to four laser diodes and four fiber outputs. The
fiber outputs can be used separately for simultaneous top and

bottom soldering applications or combined to provide up to 80
watts of average output power.

Today’s systems are easily customized to fit specific industrial

applications. For example, to solder smaller leads, smaller laser
spots are essential; hence, changing a lens to give a different laser
spot size may be required. Similarly, direct on-axis real-time
viewing of the laser spot is often necessary, and camera acces-
sories can be configured to provide this capability.

Too often in the past, the laser has been seen as an easy answer

to manufacturing problems, and the complex issues associated
with introducing a new technology such as this should not be
underestimated. A number of key system requirements must be
considered for successful industrial implementation of diode
lasers, including reliability, wavelength and ease of integration.
However, many advantages to employing lasers for manufactur-
ing also exist, and, as the industry turns the corner in 2004, laser
soldering appears at last to be in a good position to benefit from
investment in new technology.

■

Reference

1. Tim Skidmore and Karen Walters. “Optimizing Solder Joint Quality—Lead

Free.”

Circuits Assembly,

April 2000.

Dr. Tony Hoult

is a laser applications specialist in Coherent, Inc.’s Laser

Applications Center, Santa Clara, CA; (408) 764-4000; email: tony.hoult@

coherent.com.

Circuits Assembly

FEBRUARY 2004

www.circuitsassembly.com

Laser Soldering Parameters

The main objective of laser soldering is to achieve high-integrity

joints. Following is a list of parameters to consider while developing a

soldering process. It is applicable to every solder joint configuration.

•

Laser average power (in watts):

Average power controls the

rate of heat delivered to the joint. High average power is preferred to

minimize soldering time, but excess power causes vaporization and

reduces joint quality.

•

Laser pulse time/length (in ms):

Along with average power,

pulse time/length controls the amount of energy delivered by a laser to

the joint.

•

Laser pulse duty cycle (% on/off):

Pulse duty cycle modifies

the rate at which heat is delivered to the joint, giving increased control

of the process. A high duty cycle, which allows minimum soldering times,

is preferred.

•

Laser power density (intensity, watts/cm

Power density

(intensity) controls the response of the material to the laser beam and, in

conjunction with average power, generally determines the rate of the sol-

dering process.

•

Laser focus position:

Accurate positioning of the laser focus

spot is critical to ensure good joint quality and is best achieved using

precision x-y positioning tables and/or robotic arms coupled with a

CCD camera and an imaging accessory that allows coaxial viewing of

the laser beam in real time.

As the industry turns the corner in

2004, laser soldering appears at

last to be in a good position to

benefit from investment in new

technology.