Qin Y. Micromanufacturing Engineering and Technology

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also be joined with plastics. The combination of

the propert ies of these hybrid components offers

high mechanical strength, resistance to wear, high

temperature stability with, at the same time,

reduced weight and variable shape forming.

Due to the high energy density of laser radia-

tion, quick heating is possible. If necessary,

selected regions alone can be heated to limit the

total amount of absorbed energy. The tempera-

ture of the part can be measured by a pyrometer to

control the accurate joining temperature, depen-

dent on the materials and geometries used. Thus, a

stress-minimized insertion of the components is

possible and damagin g overheating does not

occur.

Transparent plates made of sapphire or fused

quartz integrated into the tool enable the irradi-

ation within the closed tool directly onto the

surface of the metal even during the forming

operation.

An essential process parameter is the joining

temperature. When too high temperatures are

applied, bubbling and discoloration occur. If the

temperature is too low, cracks open due to high

induced stress.

To reduce the cycle time a peripheral software

control for regulation of the power output is used,

which allows reducing the heating time by a time-

dependant power modulation.

A high power output is applied to reach the

intended temperature, then the power is reduced

to keep the temperature constant. An example of a

temperature/time curve is shown in Fig. 10-16.

In the present case there is a heating period of

4 seconds at 25 watts to achieve a temperature of

135



C. Afterwards the laser power is reduced to

16.5 watts.

FIGURE 10-15 Alternative irradiation strategy for non-

transparent plastics.

FIGURE 10-14 Process sequence.

CHAPTER 10 Laser-Assisted Micro-Forming 171

Plastic components can easily be formed,

which thereby enables many design possibilities.

Low density and chemical resistance are addi-

tional reasons for the increased application of this

material. On the other hand, the high strength is a

characteristic and important advantage of metals

and ceramics. The combination of the materials

results in the combination of their positive prop-

erties.

Principal fields of applications are anywhere

where this combination of properties is reason-

able. These are, for example:

1. Joining of rimless plastic eyeglasses with the

arm. Besides higher strength of the bond and

the security against loosening, new designs

are possible. No pre- or post-processing such

as pre-drilling for screwed connections is

necessary.

FIGURE 10-16 Reduction of heating time by modulation of laser power.

TABLE 10-1

Metallic Thread Insert Plastic-Ceramic Bond Plastic-Plastic Bond Plastic-Silicon Bond

– Material: steel, PMMA

– Thickness of plastic plate:

10 mm

– Screw diameter: M4

– Form closure by a bump

– Material: ZrO

; PMMA

– Thickness of plastic plate:

10 mm

– Diameter of cylinder: 8 mm

– Form closure by surface

roughness

– Additional drilling to avoid

throw-offs

– Material: Teflon, PMMA

– Thickness of plastic plate:

10 mm

– Diameter of cylinder: 10 mm

– Form closure by a groove

– Material: silicon; PMMA/PC

– Thickness of plastic plate:

3mm

– Diameter of silicon plate:

7mm

– Thickness of silicon plate:

0.3 mm

172

CHAPTER 10 Laser-Assisted Micro-Forming

2. Joining of plastic windows with metal frames

with high leak tightness.

3. Metallic pins at heavily and often loaded

hinges in plastic components, e.g. cell phones.

Besides having improved durability, the joint

also ensures higher product quality.

4. Mounting of plastic components on metal

components.

5. Plastic components with metallic inserts. The

mechanical load can be absorbed by the

mechanical part to reinforce the compound.

The main considerations of the process are:

1. Accessibility of the laser radiation to the com-

ponent to be heated;

2. Different temperature stability of both com-

ponents;

3. Maximum size of the components;

4. Geometry with positive locking.

Table 10-1 shows some exemplified bonds.

CONCLUSIONS

Warm forming with laser radiation is a promising

technique to enable new processes or enlarge the

processing limits of conventional processes. In

comparison to other production technologies like

lithography, no additional operations are neces-

sary. Elevated temperatures of the workpiece

material lead to a reduction of the yield stress

and an increase of the forming-ability. As heat

source laser radiation is applicable, making a fast

heating of selected areas of the workpiece is pos-

sible. The demand towards reproducible and high

quality process results requires the accurate mea-

surement of occurring temperatures during the

forming operation, which can be achieved by the

employment of a pyrometer. The system technol-

ogy, the process basics as well as exemplified

applications have been shown for the processes

of stamping, hot embossing and LIFTEC



REFERENCES

[1] E. Doege, St. Janssen, J. Wieser, Kennwerte f

€

ur die

Magnesiumumformung am Beispiel von AZ31 (2001).

[2] E. Egerer, T. Neudecker, U. Engel, Grundlagenunter-

suchungen zum Halbwarmmikroumformen, Tagungs-

band zum DFG-Kolloqium in SPP 1074 (2001).

[3] W. Hellwig, Spanlose Fertigung: Stanzen. Friedl,

Vieweg & Sohn Verlagsgesellschaft mbH, Braunsch-

weig, Wiesbaden (2001).

[4] S. Kleiner, Magnesium und seine Legierungen,

Feinstbearbeitung technischer Oberfl

€

achen – 6, Inter-

nationale IWF-Kolloqium (2002).

[5] W. K

€

onig, F. Klocke, Blechbearbeitung, Fertigungs-

verfahren Band 5, VDI Verlag, D

€

usseldorf (1995).

[6] H. L

€

upfert, Metallische Werkstoffe, Akademische

Verlagsgesellschaft Geest & Portig K.-G, Leipzig

(1958).

[7] A. P

€

ollmann, Prozessparameter beim Scherschneiden

von Karosserieblechteilen, Druckhaus Berlin-Mitte

GmbH, Berlin (1997).

[8] H. Schmann, H. Oettel, Metallographie, Wiley-VCH

Verlag GmbH, Weinheim (2005).

[9] K. Sieber, Werkstoffe, Fertigungsverfahren und

Maschinen der Massivumformung, VDI-Verlag

GmbH, Hamburg (1969).

[10] F. Vollersten, M. Schilf, T. Seefeld, Verbesserung der

Umformbarkeit von Aluminiumlegierungen, in: UTF

Sience 4 (2006) 1–5.

CHAPTER 10 Laser-Assisted Micro-Forming 173

Micro-Mechanical-Assembly

Hans Nørgaard Hansen, Mogens Arentoft,

Guido Tosello and Asta Gegeckaite

INTRODUCTION

This chapte r gives an introduction to micro-

mechanical assemb ly and proposes a classifica-

tion and characterization of micro-mechanical

assembly methods. Micro-mechanical assembly

is defined as assembly methods on a micro-scale,

where the relative position of components is

retained by exchange of contact forces provided

by mechanical constraints. Based on this defini-

tion, the current chapter will not deal with solid

bonding, welding, gluing, etc.

In view of the high quality and accuracy require-

ments on the mechanical assembly of miniaturized

products in the precision mechanical engineering

industry, manual methods still prevail. Manual

micro-assembly procedures are extremely demand-

ing on the human operators performing them,

time-consuming, costly and frequently they give

rise to quality problems in terms of uniformity.

Automatic procedures with a low level of flexibil-

ity have been adopted only where high product

volumes occur [1–3]. Hence, highly automated

systems for micro-assembly are not suitable for

medium/small production batches due to their

not respecting the minimum cost of manufacturing

principle. These problems are rendered more

severe by the trend towards further miniaturization

of mechanical components and by the increasing

variety of products and models.

Forms of flexibility have been developed in

classical macro-scale assembly technology which

can also be adopted in princ iple when assembling

micro-components. Assembly systems can be

adjusted to various models by mean of a modular

product design which uses easily interchangeable

product-specific system components. Further-

more hybrid micro-systems to be assembled

should be constructed on a modular principle,

dependent on the particular application, by using

standard components [1,3].

The actual mechanical assembly function

can be divided into the following constituent

functions:

Handling and positioning.

It has the function of putting two or more

objects into a particular mutual position and

orientation. Handling comprises processes of

selection and preparation of components for

composing or checking and transportation to

the following production, assembly or packag-

ing systems.

Mechanical assembl y.

It has the function of ensuring the mutual rela-

tionship between components against outside

effects. By mechanical assembly, connections

between the components can be created by

means of mechanical constraints. The assembly

process can be achieved by means of shape,

material, force, etc.

Quality control.

It has the function of ascertaining whether the

mechanical assembly process has been carried

out as specified. Checking represents those pro-

cesses by which the compone nt’s presence and

position are checked in addition to the quality

of the finished product.

CHAPTER

174

Assembly becomes particularly challenging

when dealing with micro-products. A first

approach may be to min iaturize macro-scale solu-

tions by simple downscaling of dimensions. This

approach reaches a limit with respect to obtain-

able tolerances in manufacturing processes and

handling solutions. Another natural approach is

then to try to reduce as much as possible the

mechanical manipulation of micro-components,

through a higher level of integration as compared

to that of conventional size products. This

involves a high degree of integration with the

design phase when the product development is

still in the early stage as well as the choice of

materials, which obviously determines the process

choice itself [1].

Finally it should be considered that the assem-

bly of two or more compone nts may result in a

sub-assembly which will then be subject to further

processing or assembly (Fig. 11-1).

Mechanical assembly of micro-components

presents quite a few challenges because of the

reduced dimensions [3]:

In the micro-world, submicron precision is

often required, comparable to wafer stepper

precision. This degree of precision is beyond

the calibration range of conventional open-

loop precision assembly devices used in

mechanical industry. Closed-loop strategies are

required to compensate for poor kinematic

models and thermal effects: real-tim e vision

feedback is perfectly suited for this application.

In the micro-world, forces other than gravity

dominate due to scaling effects. Surface-related

forces, such as electrostatic, van der Waal s and

surface tension forces, becom e dominant over

gravitational forces.

Manual handling of micro-parts shows the

problem of the loss of direct hand/eye coordi-

nation. It implies a series of interconnected

problems: because of the dimensions in the

micro-range, microscopes for vision and

micro-grippers for manipulation are suitable.

In order to achieve a good resolution, high mag-

nification is used, but then problems related to

restriction of field of view (smaller than the

object), very short depth of focus (an unc lear

image) and short working distance arise. Mag-

nification could be reduced, but then issues of

trade-off between field of view and resolution

emerge. Furthermore, micro-grippers have less

degree of freedom than the human hand and

lack of force feedback.

Manual handling is time consuming because

operators can handle only one or a few objects

at a time, which implies high production costs.

FIGURE 11-1 Levels of micro-assembly.

CHAPTER 11 Micro-Mechanical-Assembly 175

CLASSIFICATION OF MICRO-

MECHANICAL ASSEMBLY

METHODS

Micro-mechanical assembl y met hods can be clas-

sified according to the mechanical constraint and

the way the mat erial is processed in order to

achieve this constraint (Table 11-1). The mechan-

ical constraint is divided into two categories: pos-

sibility for disassembly or no possibility for disas-

sembly. The material conditioning characteristics

are no deformation, elastic deformation, plastic

deformation, flow and solidification. The micro-

mechanical assembly met hods listed in the table

are those typically found in the literature when

searching for micro-assembly. This implies that

methods and technologies may exist which are

not mentioned in this chapter. However, it is

believed that the most relevant technologies are

mentioned in Table 11-1. A more specific descrip-

tion of the single technologies will be given in the

following sections.

Micro-snap Fits

Micro-snap fit is a mechanical joining method,

based on the elastic deflection of joint features

on one micro-part that are inserted into a mating

feature on another micro-part, to obtain an elastic

interference.

Complicated micro-structures consisting of

multiple components can be assembled using snap

fasteners. They also have great potential when

being lifted out of plane and used as vertical

plug-in connectors.

A snap fastener consists of a mating pair of an

anchor and flexible latches. As an example, it may

have two outer latches and a central anchor. The

latches are supported on flexible beams. In its

disengaged state, the anchor and the latches can

move freely with respect to each other.

In this type of assembly only a linear movement

is required to engage the components. The relative

positioning accuracy required depends on the

absolute dimensions, but is relatively large due

to the self-aligning nature of the joint. The design

can be modified in such a way that the required

force for engagement is small. See [4–8] for exam-

ples of micro-snap fits.

Micro-screwing

Screws are components with a thread (a uniform

cross-section following a spiral or helical path)

either on the inside or outside surface. Threads

may be right handed or left handed. The screw

may be cylindrical or tapered. Cylindrical screws

need a counterpart geometry, whereas tapered

screws usually create the geometry in the counter-

part. Assembly operations involving screws

require a combined translational and rotational

movement. Furthermore a certain alignment

accuracy of the screw with respect to the counter-

part is required. The level of this accuracy depends

on screw dimensions and tolerances.

TABLE 11-1

Overview of Mechanical Assembly Methods at the Micro-Scale

Characteristic Material Constraint

Disassembly

Possible

Deformation

Elastic

Deformation

Plastic

Deformation

Flow and

Solidification

Snap fit X X

Screw X (X) X

Velcro X X

Joinery X X

Injection molding X X

Riveting, folding,

clinching

XX X

176

CHAPTER 11 Micro-Mechanical-Assembly

Very little literature exists on micro-screwing

although it is extensively used, for example in the

watch industry. ISO standar d geometries are not

necessarily scaled down to micro-scale (metric

screws according to ISO 68-1) in these applica-

tions (Fig. 11-2). Micro-screws are used in medi-

cal devices (e.g. hearing aids) as one of the

simplest fastening components. In some of the

products, the screw has another functionality,

namely creating an electrical connection.

Micro-Velcro

Micro-Velcro is a micro-mechanical fastening

system, based on silicon micro-machining tech-

nology, which results in a strong, permanent bond

without chemical adhesives (Fig. 11-3). The

working principle is based on arrays of micro-

mechanical mating structures, which act as

mechanical adhesives.

The joining principle of micro-Velcro is based

on the matching of silicon wafers with highly

density micro-structured surface, with an areal

density of approximately 200,000 units/cm

The micro-structures on tw o identical surfaces

will self align and interlock with each other under

application of adequate external pressure. A ten-

sile strength per unit interlocked area of the order

of 0.2 MPa has been achieved [10].

The principle of bonding is a button snap, or a

zipper, but in a two-dimensional configuration.

The bonding principle is depicted by the sche-

matic cross-section in Fig. 11-3. Under applica-

tion of adequate external pressure, the tabs of the

structures deform and spring back, resulting in an

interlocking of the two surfaces. In this way, a

permanent bond is achieved. Other examples are

reported in [11,12].

FIGURE 11-2 Example of micro-screw with non-ISO

dimensions and geometry [9].

FIGURE 11-3 Principle of a micro-Velcro assembly. Left: two initial parts. Right: assembled unit.

CHAPTER 11 Micro-Mechanical-Assembly 177

Micro-joinery

Micro-joinery is the fabrication and assembly of

micro-joints to realize three-dimensional micro-

structures. Each micro-joint contains two or more

mating surfaces which join the various parts

together as a single unit. This technique involves

primarily silicon but virtually any single crystal-

line material of virtually any crystalli ne orienta-

tion (GaAs, Ge, quartz, metals, etc.) is suitable

[13]. In macro-scale mechanical assembly this

technique is well known. It is therefore also appli-

cable for metallic, ceramic or polymer micro-

components providing that adequate processing

technologies exist and can be applied. A critical

point is, of course, the tolerance s of the parts. On

the one hand, they should ensure mating, and on

the other hand, they should be as small as possible

in order to secure the assembly. The surface area

to volume ratio favors the strength of the joint on

micro-scale but it also is a potential challenge in

terms of actual assembly.

The dovetail micro-joint is a particular adap-

tation of the micro-joinery concept and it is com-

monly used in 3D (x, y, z) positioning devices

requiring linear translation – see Fig. 11-4 for an

example.

The slot joint is similar in function to the dove-

tail micro-joint in that it also has constrained

translation, but method and geometry are slightly

different. The slot joint has a rectangular-shaped

cross-section (Fig. 11-5). A number of techniques

are suitable for the fabrication of a slot joint:

micro-milling, sawing, LIGA or anisotropic etch-

ing of silicon wafers. Finger joints are interlocking

structures that feature a periodic assembly of mat-

ing fins as shown in Fig. 11-5. Finger joints are

used to attach two substrates rigidly together,

paying attention to the fact that large mating sur-

faces of the finger impart considerable strength

and stability to the joint.

FIGURE 11-4 Dovetail assembly.

FIGURE 11-5 Slot joint (left) and finger joint (right).

178 CHAPTER 11 Micro-Mechanical-Assembly

Micro-injection Molding

Micro-injection molding is a manufacturing tech-

nique that can be us ed not only for the production

of monolithic micro-structures but also for the

assembly of hybrid structures. As components

and functional structures become smaller, they

cannot be regarded in terms of steps in a single

process, but as an integrated production concept.

A process called micro-assembly injection mold-

ing has been developed and combines the joining

of hybrid elements with the generation of func-

tional structures [14,15].

Micro-injection molding allows the produc-

tion of movable micro-structures by using

incompatible polymers, soft/hard combinations

of materials, the generation of fluidic hollow

structures by lost core technology and the over-

molding of w ires and optical systems l ike optical

fibers.

The differences of materials used, the temper-

ature layout of the process and the mechanical

strain of inlay parts play a fundamental role. Most

of all, the precision in positioning of inlay parts

and the demolding of hybrid struc tures have a

relevant importance. For these reasons, a mold

technology with flexibility, precision and special

processing equipment is required. The mold is

characterized by using a special system for

positioning of inlay parts and complex sensor

equipment for measuring pressure and tempera-

ture, which are relevant process parameters.

Thus, a new mold concept has to consider the

possibility of various thermal processes, such as

heating the cavity before injection, cooling down

again before de molding. It has to be characterized

by acceptable cycle times and the possibility to

evacuate the cavity. Figure 11-6 illustrates hybrid

micro-structures produced by injection molding.

Overmolding of other polymer parts is used in

two-component micro-injection molding, for

example for the creation of molded interconnect

devices [17].

Micro-riveting, Folding and Clinching

Riveting, folding and clinching are mechanical

assembly processes based on plastic deformation

of the materials involved. This excludes brittle

materials from these particul ar processes al-

though MEMS-based riveting has been reported

[18]. In macroscopic assembly processes these

technologies are used in metal joining/assembly.

They require the use of molds/dies and tools and

are based upon sheet metal operations: reference

can be made to a relevant chapter for the basis of

these technologies. Figure 11-7 illustrates a micro-

rivet which was produced by a micro-cold forging

operation.

FIGURE 11-6 Hybrid (metal/polymer) structures manufactured by injection molding. Material: PS (left) and PA66 + GF50%

(right). Metal foil thickness up to 100 mm [16].

CHAPTER 11 Micro-Mechanical-Assembly 179

SYSTEMATIC APPROACH

TO MICRO-MECHANICAL

HANDLING AND ASSEMBLY

The handling of the small parts has been studied

for almost half a century and different micro-

handling principles are summarized in [1]. A clas-

sification scheme for the quantified analysis of

micro-gripping principles has been proposed in

[3] and describes basic features of the gripping

operation on micro-scale. Different handling clas-

sification schemes have been introduced in recent

years. Different realiz ations of micro-grippers

have been reported (their diversity is large)

addressing different handling situations, materi-

als and dimensions.

However, when developing solutions for

micro-mechanical handling and assembly, a sys-

tematic approach is beneficial. Figure 11-8 illus-

trates the contents of a proposed methodology.

Object characterist ics (material, weight, dimen-

sion, geometry) are considered as the basic infor-

mation of the problem. Depending on what

should be done with the objects (referred to as

functionality) a choice of gripping principle and

assembly method can be made. The description of

the desired functionality is important due to the

fact that the resulting feedback may also involve a

redesign of the component to make it more suit-

able for the assembly operation.

It is not t he methodology t o automatically

result in an optimized overall handling and

assembly process. However, by considering all

the points in the methodology a characteriza-

tion of the entire situation is possible and an

identification of the most important st eps in

the process is possible. The systematic consider-

ation of what exactly is needed in terms of

operations will hopefully result in gripper

designs that are tailor-made. The next section

will illustrate how the methodology has been

applied.

APPLICATION EXAMPLE:

MECHANICAL ASSEMBLY

OF PUSH BUTTON PARTS

As an example of mechanical assembly, a hear-

ing aid push button will be taken. The push

button con sists of seven differe nt parts both geo-

metrically and in respect of the material (see

Fig. 11-9). All parts have an axis-symmetrical

geometry but different diameters and shapes.

The screw is shown in detail in Fig. 11-2. Different

geometrical dimensions of the pa rts are up to

FIGURE 11-7 Micro-rivet in Ag.

180 CHAPTER 11 Micro-Mechanical-Assembly