Qin Y. Micromanufacturing Engineering and Technology

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2 mm, but dimensions of the functional structures

are less than 1 mm, therefore these parts are

known as micro-objects. Assembly of these parts

is usually done by putting parts 1–3 onto each

other, then screwing a screw through part 3 to

part 1. A lid with two wires is placed on top and

fastened by thermal heating and gluing, for seal-

ing purposes: the lid and wires are not used in this

example. Presently these assembly operations are

performed manually or on semi-automatic sta-

tions involving manual labor.

The requirements of the handling and assembly

process can be summarized as follows:

The assembly operations should be performed

using a small standard industrial robot.

The parts (all axi-symmet rical) need to be

aligned with respect to each other.

The screwing process must be achieved by a

combined rotational and translational move-

ment using a certain torque. In this process,

either the screw or its counterpa rt should

be moving while the other is kept in a fixed

position.

It was chosen to work with small pallets as

carriers for the components because the industrial

company already ha d good experience wi th this

type of system. The parts were therefore placed in

separate fixtures and then picked and placed dur-

ing the assembly process. Two types of tools were

needed for handling and assembly: one capable of

executing a combined rotational/translational

movement and one capable of picking and releas-

ing axi-symmetrical parts.

Two assembly scenarios were considered

(Fig. 11-10): In Fig. 11-10(a) a screwdriver is fixed

and the parts are manipulated one by one using a

mechanical gripper. In the first step, the screw is

FIGURE 11-8 Systematic approach to micro-mechanical handling and assembly [9,19].

FIGURE 11-9 Schematic view of push button parts: 1 –

knob, 2 – spring, 3 – holder and 4 – screw. Scale on left

picture in mm.

CHAPTER 11 Micro-Mechanical-Assembly 181

fixed in the scre wdriver by means of vacuu m.

Subsequently, the remaining parts are fitted over

the screw one by one, and finally the screwing

process is perfor med. In the second scenario

(Fig. 11-10(b)) three of the parts are picked and

placed on top of each other by a mechanical grip-

per. Then a screwdriver is used to pick and screw

the small screw into place and to perform the

screwing operation [9].

The design of the mechanical gripper is a sub-

ject of high importance. On the one hand, the

gripper should match the shapes of the objects –

preferably of all the objects – and on the other

hand, a custom-made design for each part would

ensure a better picking and releasing operation.

As a compromise, a two-finger gripper design

was investigated (see Fig. 11-1 1). Various angles

of the slots as well as surface textures were

FIGURE 11-10 Assembly scenarios of push button [20–22].

FIGURE 11-11 Mechanical gripper. V-groove design (left) and mounted on robot (right).

182 CHAPTER 11 Micro-Mechanical-Assembly

investigated in order to optimize the design. It

was conclude d that a flexible design with a V-

groove would accommodate all the various dia-

meters of the parts. Furthermore, the use of

micro-structured surfaces facilitates the release

of objects, especially in case of the plastic parts,

which have weights ten times smaller than that

of the screw [23] .

The detail-designed screwdriver (Fig. 11-12 )is

composed of two different mec hanisms. The first

is an air suction-based mechanism [3] (11 shows

where it has to be connected) for picking up the

screw (1) and holding it in the required position

while transporting it. The vacuum system consists

of a pump, tubes and a manipulator (not shown in

the figure). The second mechanism is a screwing

mechanism that is composed of a screwdriver (2),

a scre wdriver shaft holder, bearings, a hexagonal

key, a coupling and a motor with a gearbox (9).

The remaining parts are designed for supporting

the screwdriver. The corpus consists of two parts:

4 and 11. The corpus has a chamber for air and air

connection place (3), the motor is kept in the

corpus, and fastened to a motor holder (10). The

screwdriver shaft holder and the motor are kept in

place by two screws. Four screws connect both

parts of the corpus and the motor holder. The

corpus is fastened by (11) holes on the sides, by

which it is held by a robot hand [24].

With the use of the described gripping and

screwing device, the relative limited accuracy of

the robot was compensated for. The described

assembly process was successfully tested in a

laboratory environment, and is currently consid-

ered for industrial implementation.

CONCLUSION

Mechanical micro-assembly methods are among

the most highly relevant in micro-manufacturing.

They allow the assembly of different materials,

primarily non-silicon, and potentially they can

incorporate possibilities for disassembly. The

challenges related to micro-mechanical assembly

include the almost total lack of guidelines (i.e.

standards, best practice, etc.) and the need for

tailor-made tools for the processes. This often is

used as an argument against the automation of

such processes.

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FIGURE 11-12 Detailed screwdriver design. 1 – micro-screw, 2 – shaft, 3 – vacuum connection place, 4 – corpus,

5 – holder, 6 – bearings, 7 – shaft, 8 – shaft coupling, 9 – motor and gearings, 10 – motor holder and 11 – corpus.

CHAPTER 11 Micro-Mechanical-Assembly 183

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184 CHAPTER 11 Micro-Mechanical-Assembly

Laser Beam Micro-Joining

Felix Schmitt and Alexander Olowinsky

INTRODUCTION

The joining processes in electronic device

manufacturing are today still dominated by con-

ventional joining techniques such as press fitting,

crimping and resistance welding. Laser beam join-

ing techniques have been under intensi ve investi-

gation and subsequently new processes for mass

manufacturing and high accuracy assembling

have been established. With the newly developed

SHADOW



welding technology, technical

aspects such as tensile strength, geometry and

precision of the weld can be improved. This tech-

nology provides the greatest flexibility in weld

geometry with a minimum welding time a s well

as new possibilities in using application adapted

materials. Different parts and even differe nt

metals can be joined by a non-contact process.

The application of a relative movement between

the laser beam and the part to be joined at feed

rates of up to 60 m/min produces weld seams with

a length of 0.6 mm to 15.7 mm using a pulsed Nd:

YAG laser with a pulse duration of up to 50 ms.

Due to the low energy input, typically 1 J to 6 J, a

weld width as small as 50 mm and a weld depth as

small as 20 mm have been attained. Thi s results in

low distortion of joined watch components.

In the field of micro-production a variety of

materials with individual product-specific dimen-

sions are commonly used. Especially, the

manufacturing of hybrid micro-systems, built up

from different functional groups, affords variable

joining technol ogies tailored to the specific

demands of each component or material combi-

nation. Soldering with metal solder alloys is an

established and well-known proces s in electronic

industries [1].

LASER BEAM SOLDERING

During the soldering process a liquid phase is

caused by melting of a solder alloy or by diffusion

processes within the intermediate layer. In princi-

ple, the joining process is based on interaction

reactions between the joining partners and the

melted solder. Therefore, a direct, oxide- and con-

tamination-free, contact between the metal sur-

faces of the joining partners and the solder alloy

is one of the most important process require-

ments. If the melting temperature of the addi-

tional material is below 450



C (840



F) the

process is called soldering, while when above

450



C the process is called brazing. Characteris-

tic features of soldering include [2]:

The melting temperature of the joi ning partners

is higher than the melting temperature of the

solder alloy.

In most cases the service temperature of the

assembly must be lower than the melting tem-

perature of the solder alloy. In a diffusion sol-

dering process, called transient liquid phase

(TLP) bonding, the service temperature can be

higher than the soldering process temperat ure.

Prior to the joining operation, the surfaces have

to be cleaned to remove oxides and organic

films. The use of a flux can avoid prior proces-

sing. However, there are some constrictions

associated with the use of flux, e.g. the residues

that they leave behind, which are often corro-

sive and can be difficult to remove.

CHAPTER

185

Complex assemblies can be produced with low

distortion, high fatigue resistance and good

resistance to thermal shock.

Joints tend to be strong if well filled, unless

embrittling phases are produced by reaction

between the solder alloy and the components.

By using a broad variety of solder alloys it is

possible to match the solder with the required

process temperature and to reduce the melting

temperature by using elements with a low melt-

ing point (e.g. indi um, bismuth).

Compensation and gap bridging can be

achieved by using an additional filler material

as solder alloy.

For a molten solder alloy to wet and bond to a

metal surface, the surface has to b e free from

non-meta llic surface films. Despite a pr e-

clea ning process and ensuring this condition

at the beginning of the process, significant oxi-

dation will occur if the components are heated

in air. An active flux fulfills the following func-

tions [2]:

T Removal of oxides and other films on sur-

faces by either chemical or physical means.

T Protection of the cleaned joint from oxida-

tion during the soldering process.

T Wetting the joint surfaces, but being dis-

placed by the molten solder as the latter

spreads.

T Reducing the surface tension between the

solder alloy and the joint surface, thereby

enhancing spreading.

The heating cycle invol ves four important para-

meters: the heating period with heating rate

and dwell time for heating, the peak soldering

temperature, the dwell time above the melting

point of the solder alloy and the cooling rate

(Fig. 12-1). In general, it is desirable to use a

high heating rate but the maximum heating rate

is normally constrained by the form of the

energy input. By means of laser energy and its

high energy density it is possible to realize a

maximum heat rate. The dwell time for heating

is necessary for the evapo ration of vapor and

constituents of the flux and for the unifor m

heating of the joining partners up to the wetting

temperature. This temperature is below the

melting temperature of the solder alloy. The

soldering temperature should be such that

the solder alloy is certain to melt, but at the

same time the solder alloy should not be ov er-

heated so that it degrades through the loss of

FIGURE 12-1 Heating cycle for soldering.

186 CHAPTER 12 Laser Beam Micro-Joining

constituents. The peak temperature is normally

set at about 20–30



C above the melting point.

The minimum time that the joint geometry is

held at this temperature must be sufficient to

ensure that the solder alloy has melted over the

entire area of the joint. Extended holding times

tend to result in excessive spreading of the mol-

ten solde r alloy, possible oxidation gradually

taking place, and deterioration of the proper-

ties of the parent materials. The cooling stage of

the cycle is not controlled by the operator but

normally governed by the thermal mass of the

joint geometry. For laser processing it is very

fast because of the instantaneous switch-off of

the laser power, resulting in a fine-grained

micro-structure of the joint.

Apart from light beam soldering and electron beam

soldering, laser beam soldering is a soldering tech-

nique using radiation as an energy source (DIN

8505 1979). In contrast to other conventional selec-

tive soldering techniques, laser beam soldering

features a contactless, temporally and spatially

well-controllable energy input. Because of these

characteristics, laser beam soldering is predestined

for joining tasks where miniaturization and

reduced thermal and mechanical stresses are

required. Special features of laser beam soldered

joints are fine-grained micro-structure and a low

amount of intermetallic phases due to the high

heating and cooling rates of this process. In princi-

ple, laser beam soldering is characterized by tem-

porally and spatially selective energy input by

surface absorption in the joining area, successive

heat conduction and interface processes. The join-

ing process is determined by characteristics of the

laser beam source, the chosen process parameters

and the thermo-physical properties of the joining

partners.

The first tests on soldering with laser radiation

were conducted in 1974 by C.F. Bohmann [3].

Here, a continuously emitting CO

laser was used

for selective contacting of electronic components,

the influence of laser power, irradiation time and

geometry of laser beam interaction zone on the

quality of the joints being explored. Although dif-

ferent research teams were working on the indus-

trial implementation of this process, the CO

laser

did not become a widely accepted tool for solder-

ing tasks despite the good automation possib ilities

[4,5]. There are some technological and econom-

ical reasons which are inhibiting the use of CO

lasers as a laser source for soldering applications.

Widely used basic materials in electronic produc-

tion (e.g. FR4) have an absorption of more than

90% at the emission wavelength of CO

lasers

(l = 10,600 nm), but soldering alloys (e.g. tin-

lead) have a reflection of about 74% at the same

wavelength [6]. In consequence, the risk of burn-

ing or partial carbonization of the circuit boa rd

is high by primary or diffused scattered laser

radiation. Apart from these process-specific

drawbacks, the investment and operation costs,

maintenance effort and dimensions of this laser

source are not an advantage compared to the use

of technological alternatives.

Nd:YAG lasers were used for the first time for

soldering applications in the 1980s [7,8]. Com-

pared to CO

lasers, these laser sources have dif-

ferent positive features and emit light at near

infrared (l = 1064 nm). Apart from smaller

dimensions, the shorter emission wavelength

enables flexible and cheaper beam shaping and

guidance. Here, fiber optics is used for beam guid-

ance and optical components need not be made

from materials such as germanium, zinc-selenium

or cadmium-tellurid but can be made of cheaper

optical glasses. Metallic materials have a higher

absorption in the near infrared, resulting in a

more efficient and reproducible proces s. The main

applications for laser soldering systems based on

Nd:YAG lasers are electr ical and mechanical

joints from sectors which require a greater level

of reliability. Here, precision applications are

known from civil fields, e.g. computer technol-

ogy, automotive, aviation, aerospace, but there

are also applications from military fields [6,9].

Because of high investment and operational costs,

even laser beam soldering with solid-state lasers is

only established in smal l market segments com-

pared to competing selective soldering technolo-

gies.

Since the development of high power diode

lasers, laser beam soldering has become increas-

ingly more important in industrial applications.

CHAPTER 12 Laser Beam Micro-Joining 187

Diode lasers feature a simple layout, a high elec-

trical efficiency factor and small dimensions and

are appropriate for manifold industrial applica-

tions on this accoun t. In combination with not

requiring maintenance, being of simple operation

and having long lifetimes, these laser sources ful-

fill the requirements of industry due to providing

an economic operation by lasers [10] .

The latest developments in laser technology

have resulted in fiber lasers becoming a versatile

tool for laser-based production processes. Even if

they are currently not used for soldering, very

often it can be seen that they will penetrate laser

beam soldering applications within the next few

years. Due to their excellent beam quality, only

these laser sources provide minimal focus geome-

tries meeting the requirements for further minia-

turization. Furthermore, these laser sources

feature continuous emission, small emissio n

wavelengths (l = 1030 nm) and high anticipated

lifetimes, combined with small dimensions.

Table 12-1 shows the important characteristics

of these four laser sources for soldering applica-

tions.

Although the beam quality of high power diode

lasers cannot be compared to that of conventional

laser sources, these laser sources excel at flexibil-

ity. The decisive factor in this is the dimension

ratio between the single different laser beam

sources. The beam generating dim ensions of a

high power diode laser are smaller by a factor of

100 compared to an Nd:YAG laser at the same

power. For the CO

laser this differen ce is

even greater (a factor of 1000). Compactness of

semiconductors and the higher efficiency resulting

in smaller and low power consumptive peripheral

devices are responsible for the small system

design. This saves space in the production envi-

ronment and increases the mobility of these laser

sources. Therefore, it is possible to integrate diode

lasers directly into production cells. With respect

to economical aspects, modern high power diode

lasers are an attractive alternative to Nd:YAG and

lasers. Investment costs related to the optical

output are at the lower range of competitive laser

sources. Due to the epitax ic layout, diode lasers

offer the possibility for cost-efficient mass pro-

duction. Hence, forecasts see a decreasing price

for diode bars so that high power diode lasers will

gain an advantage compared to other laser beam

sources [10]. With an electro-optical efficiency of

about 50%, diode lasers work much more effi-

ciently compared to Nd: YAG (<17%) and CO

lasers (<15%), resulting in lower power con-

sumption as well as in required cooling power:

this influences directly the operational costs.

Diode lasers work almost maintenance free

because there are no components wearing out

(e.g. flash lights in Nd:YAG).

Due to the demands for high quality soldered

joints, many process control mechanisms have

been examined over the last few years. An essen-

tial requirement for laser beam soldering is the

stability of the material and geometry conditions

due to the energy input into the materials being

independent of environmental conditions. Com-

pared to other selective soldering methods, laser

beam soldering enables temporally and spatially

TABLE 12-1

Comparison of Different Laser Sources for Laser Beam Soldering

Diode Laser Fiber Laser Nd:YAG Laser CO

Laser

Dimension (ratio) 1 1 100 1000

Electro-optical efficiency 40–50% 30% 3–17% 10–15%

Emission wavelength 800–1000 nm 1030–1090 nm 1064 nm 10,600 nm

Lifetime >100,000 h >100,000 h >1000 h 10,000 h

Maintenance Low maintenance Low maintenance 200–1000 h every 500 h

Investment/power 10–50/W 15–30/W 50–100/W 10–50/W

Beam guiding (fiber optics) yes yes yes no

188

CHAPTER 12 Laser Beam Micro-Joining

adapted energy input by pyrometric process mon-

itoring. The development of a closed-loop feed-

back of process informat ion by thermal radiation

gives the possibility of process control.

There are numerous different application-

specific solutions available commercially for laser

beam soldering machines. In principle, they are

based on flexible beam shaping and guidance

using galvanometric scanners or axis systems.

For fiber-guided systems the processing optics

are moved but there are also systems where the

entire laser beam source is being moved. In

Fig. 12-2 a production cell and laser process-

ing optics are shown based on a galvanometric

scanner.

The machine is designed for the laser beam

soldering of an automotive micro-electronic mod-

ule (an alternator regulator realized in thick-film

technology) with solder pads printed on an alu-

mina substrate (Fig. 12-3). The housing has seven

terminal leads to be soldered to the substrate. The

entire alumina substrate is glued by a heat con-

ducting adhesive to an aluminum base- plate

[11,12].

The diode laser system has a maximum optical

output power of 500 W, which can be modulated

by controlling the pump current. The collimated

laser beam passes through a galvanometer scan-

ner and is focused by an f-theta lens onto the lead/

solder pad area to generate the joint. The circular

FIGURE 12-3 Automotive microelectronic module in a plastic housing – alternator regulator. Major components of the

production cell are a fiber coupled, continuous wave (cw) diode laser system and a processing head with integrated

pyrometric and power sensors.

FIGURE 12-2 System design for laser beam soldering based on a galvanometric scanner: Production cell (left) and laser

processing optics (right).

CHAPTER 12 Laser Beam Micro-Joining 189

focus geometry of the laser beam is aligned to the

center of the semicircle at the end of the terminal

lead (Fig. 12-4). The working distance between

the optics and the laser beam interaction area is

about 80 mm. With an image projection ratio of

1:2 the minimum focal diameter is 1.2 mm, which

is double the fiber core diameter of 0.6 mm.

Thermal radiation emitted from the surface

follows the beam delivery system of the galva-

nometer scanner and passes through a dichroic

mirror, which is transparent for this wavelength

range. After passing the dichroic mirror the ther-

mal radiation is focused by a lens on a photo

detector (Ex. InGaAs, peak wavelength 2.3 mm).

The output signal of the detector is conditione d by

a logarithmic amplifier circuit. The integrated

pyrometric sensor is conditioned for laser beam

applications with process temperatures in the

range of 150



C, e.g. welding of plastics or solder-

ing. The pyrometric sensor is calibrated by means

of a standardized black body and the response

time of the sensor is about 1 ms at 150



The surface of the lead/solder pad area is

imaged onto a CCD camera via a deflecting

mirror.

Apart from interconnection requirements, a

high production rate has to be ensured for the

process to remain attractive for mass production.

For this reason the total process period, especially

the irradiation time, has to be as short as possible.

However, to achieve an adequate solder joint with

reduced irradiation time the laser power has to be

increased. To avoid the hazard of superheating,

the laser power has to be limited and controlled.

Therefore the thermal radiation from the interac-

tion zone is detected and analyzed in more detail.

In a series of experiments the following features

could reproducibly be observed in the recorded

pyrometric signal. A typical profile is presented

in Fig. 12-5, where the laser is switched on at time

t = 0.2 s. At point A the reduction of the ascending

slope indicates the initial activation of the applied

adipic acid. The second change in the pyrometric

signal at point B is related to the onset of localized

melting of the solder pad and outgassing of vola-

tile components. Due to the continued energy

input by the laser beam, the terminal lead reaches

the wetting temperature (point C). In the next

phase there is a sudden improvement in heat dis-

sipation due to the wetting of the terminal lead,

which often results in a temperature decrease

(points C to E). At point D a gas bubble consisting

of volatile components leaves the molten solder.

The variation of the signa l curve following point E

is induced by self-optimization of the surface

FIGURE 12-4 Solder joint configuration (dimensions in

mm).

FIGURE 12-5 Pyrometric signal detected during

soldering; irradiation time: 1000 ms.

190 CHAPTER 12 Laser Beam Micro-Joining