Bonding Wires

         

The integrated circuit is connected to the leads of its package by bonding wires, which are very fine wires composed of  high-purity gold, aluminum, or copper.  Wirebond reliability depends greatly on the proper choice of the bonding wire, especially in advanced packaging technology which requires special loop profiles and extremely small bond spacing (fine-pitch bonding).

       

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Gold Bonding Wire

The first consideration in choosing a bond wire is the type of package it will be used for. Gold wire can't be used in hermetic packages because it won't be able to withstand the high temperature of hermetic sealing. Aluminum wire is the standard choice for hermetic assembly. For plastic packages, however, gold wire is the more logical choice because it is faster, easier to use, and therefore more cost-effective.
   

The diameter of the wire is the next important consideration. Thinner wires will be required by circuits with smaller bond pad openings, while circuits that draw large currents or require thermomechanical robustness would do better with thicker wires.

                

Another consideration when choosing a bonding wire is its tensile strength. The wire will be subjected to a lot of tensile stresses throughout its lifetime, e.g., during bonding itself, during encapsulation, during board mounting, during usage. Needless to say, the higher the tensile strength the better.
  
The
elongation property of the wire is also an important consideration in the selection of the wire. Wires with higher elongation are more difficult to control during loop formation at wirebonding. Thus, it is better to choose a wire that doesn't elongate much during the bonding process.
  
The last major consideration is the length of the
heat-affected zone of your wire. When the end of the wire is melted by flame-off to form the free air ball prior to bonding, the high temperature enlarges the grain structures of the zone closest to the ball. These larger grain structures are more vulnerable to shearing stresses that cut across the wires. Wires with a longer heat-affected zone can not be used in low-loop wirebonding because the heat affected zone may be subjected to the shearing stresses of loop formation. Normally the wire manufacturer will indicate whether the wire is for low-loop or for high-loop applications.

   

Copper wire is becoming one of the preferred materials for wirebonding. Copper wire of smaller diameter can achieve the same performance as gold wire of bigger diameter.  Copper wire, needless to say, is also more economical than gold wire.

   

With proper set-up, copper wire can be successfully wedge-bonded and be used as alternative to aluminum wire, especially in applications where higher current-carrying capacity is needed or complex geometry problems are encountered.

  

Copper wire is harder than gold and aluminum, so it has a higher tendency to contribute to die damage if the bonding parameters are not put under tight control. It is also inherent for copper to oxidize, which if left unchecked can lead to storage and shelf life issues.

   

Table 1. Properties of Various Wire Types 

Property

Cu

Au

Al

Ag

Electric Conductivity (%IACS)

103.1

73.4

64.5

108.4

Thermal Conductivity (W/m K)

398.0

317.9

243.0

428.0

Thermal Expansion Coeff (mm/m K)

16.5

14.2

23.6

19.0

Tensile Elastic Modulus (GPa)

115

78

62

71

    

Table 2. Properties of Copper Wires from Semiconductor Packaging Materials

Diameter (in.)

Hard Wire

Annealed Wire

Elongation (%)

Tensile Strength (g)

Elongation (%)

Tensile Strength (g)

0.0007

0.5 - 4

10 - 20

6 - 20

5 - 12

0.001

0.5 - 4

20 - 30

10 - 25

10 - 20

0.00125

0.5 - 4

35 - 45

10 - 25

15 - 25

0.0015

0.5 - 4

45 - 75

10 - 25

25 - 35

0.002

0.5 - 4

80 - 120

10 - 25

45 - 55

0.003

0.5 - 4

200 - 270

10 - 30

95 - 115

0.004

0.5 - 4

350 - 450

10 - 30

175 - 225

0.005

0.5 - 4

600 -700

10 - 30

260 - 310

0.010

0.5 - 4

2200 - 2600

10 - 30

1040 - 1240

  Note: Other specs available upon request: info@sempck.com

      

Table 3. Examples of Bonding Wires from Mitsubishi Materials      

Type

Diameter (um)

Weight (mg/20cm)

Breaking Strength (g)

Elongation (%)

MGS-20

20+/-1

1.1-1.34

3-7

2-7

MGS-23

23+/-1

1.46-1.75

4-8

2-7

MGS-25

25+/-1

1.75-2.05

7-11

2-7

MGS-28

28+/-1

2.21-2.55

8-13

2-7

MGS-30

30+/-1

2.55-2.92

10-15

2-7

MGS-32

32+/-1

2.92-3.3

12-17

2-7

MGS-35

35+/-1

3.5-3.93

13-20

2-8

MGS-38

38+/-1

4.15-4.61

15-26

2-8

MGS-40

40+/-1

4.31-5.35

17-30

2-8

MGS-50

50+/-1

6.98-8.2

30-50

2-10

 

Table 4. Examples of Bonding Wires from Tanaka      

Wire Type

Manufacturer's Description

GMG and GMH Gold Wires

Have greater strength and resistance to vibration than conventional products; long loops (4 mm and more) and very short loops (1 mm or less) are more reliable, providing finer pitch and reduced costs

GPG Gold Alloy Wires

Finer wire due to superior mechanical strength; greater bonding strength; smaller deformed ball diameter; high bonding reliability; can be used with conventional ball bonders

TABW Aluminum-1%Silicon Wires

Stable quality with no curls or blemishes; excellent purity, with no gas oxide absorption; ideal for hermetically sealed packaging where high reliability is demanded

TANW Power Aluminum Wires

Improved anti-corrosive properties; smooth, clean, unscratched surfaces; high purity with good bonding properties

            

Table 5. Examples of Bonding Wires from K&S/AFW      

Wire Type

Manufacturer's Description

AW-21

Gold wire for high loop applications, i.e., discrete and high-power components

AW-29

Gold wire for standard loop applications, e.g., SOT, DIP, SOP, PLCC

AW-14

Gold wire for low loop applications, e.g., QFP, TSOP, TSSOP, TQFP, BGA, PGA

AW-99

Gold wire for advanced fine-pitched packages

AW-6

Gold alloy bumping wire for stacked as well as other applications where elimination of coining and precision are key requirements

FP-2

Gold alloy wire; high tensile strength for enhanced reliability

ALW-29C

Aluminum wire; corrosion-resistant for wedge bonding

ALW-29S

Aluminum-1% Si wire; small-diameter aluminum wedge bonding wire

ALW-49P

Aluminum  high-purity wedge bonding wire

   

See Also:  Semiconductor Matls IC ManufacturingWirebonding

 

   

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