Surface finish test method project...
23
Copper Wire Bonding Draft Proposal iNEMI November 2009
Transcript of Surface finish test method project...
Copper Wire Bonding Draft
Proposal
iNEMINovember 2009
Background• Using gold wire as the electrical interconnection
between the integrated circuit and the corresponding PWB substrate is the most common and well understood material; however the use of Cu wire is increasing
• Studies indicate that copper wire can achieve greater mechanical stability than gold wire
• Standard bond strength tests such as the wire pull test and the ball shear test have demonstrated that copper ball bonds exhibit 25%-30% higher readings than comparable gold ball bonds
2
Background (con’t.)• Copper wire bonds can be so strong that the wire
itself does not break during wire pull testing, resulting instead in bond pad metal lifting.
• Using Cu wire has some distinct advantages as well as disadvantages (see next slide)
• Cu wire bonding has been explored at the IC interface and the substrate interface using “standard” PWB materials
• New PWB materials are being introduced at an increasing rate and little information is known about the impact of Cu wire bonding on the reliability of these new material systems
3
Advantages• Lower cost• High thermal conductivity• Low electrical resistance• Higher breaking load (smaller
wire diameters higher density)
Disadvantages• Tends to undergo oxidation at
relatively lower temperatures• Hardness requires bonding
parameter (bond force and ultrasonic energy in particular) optimization to achieve effective bonding without causing damage
• Failure analysis more difficult• Not as well-understood as gold
ball bonding technology• Limited reliability data on new
PWB materials
Background (Copper Wire Bonding)
4
Substrate Damage & the Wire Bonding Process
General Observations: Damage• Studies are difficult to do because damage usually
occurs at extremely low frequency under the same bonding conditions, requiring large sample sizes, carefully designed experiments, and appropriate statistical analyses to understand the process.
• The fracture process of PWB substrates cannot be easily modeled because the dynamics and the multiplicity of the variables in the wire bonding process yield too many process variations
• Low levels of damage makes modeling virtually impossible without a major investment in tools and collection of extensive amount of materials’ data
Product Design Guidelines• Limits of bonding method to be used• Metallization effects on reliability• Implications (variability) of high volume
manufacturing• Relative hardness of metal and its oxide• Wire and metallization hardness• Formation of intermetallic compounds• Electrochemical nature of metal couples• Chemical reactivity of metals
Characteristics of Bonding Methods• TC (Thermocompression)
– Temp 300°C - 400°C– Time 50 – 200ms– Pressure 40g – 100g (1mil
Dia.)– Au or Cu wire
• US (Ultrasonic)– Temp Room temperature– Time 200 – 250ms– Pressure 20g – 50g (1mil Dia.)– Ultrasonic Energy 2 – 5W @
~60KHz– Al wire
• TS (Thermosonic)– Combination of Ultrasonic and
Thermocompression– Temp <150°C– Time 20 – 200ms– Pressure 20g – 80g (1mil Dia.)– Ultrasonic Energy 2 – 5W at
~60KHz– Fastest >10 wires per Sec.– Most Automated and Popular
>75% of total bonding– Au or Cu wire
Advantages• TS (thermosonic)
– Less damage than US
• TC (thermocompression)– Least amount of damage
• US (ultrasonic)– Room temperature bonding– Finer bond pitch (< 60 µm)– Large wires (25 - 500 µm)
Disadvantages• TS (thermosonic)
– Higher forces than TC– More damage than TC
• TC (thermocompression)– High bond temperature– Thermal degradation of PWB
• US (ultrasonic)– Cu wire bonds require heat – Slower than TC or TS– Increased damage potential– Special tools (i.e.. wedges)– Alignment is critical
Comparison of Bonding Methods
Cause of Damage & Symptoms • Bonder Settings• Metallurgies• PWB Materials• Failure Symptoms
– Marginal (electrical leakage detected)– Gross (divots in the substrate; broken (cut) wires)– Sub-marginal (detection requires thermal cycling
and/or other stress tests)
Bonder Settings• Bonding Temperature / Ambiguous (both High
and Low can increase damage)• Tool to Pad Impact Force / Critical• Bonding Force control is critical
– Forces too high or too low harmful– Both require good clamping / fixturing
Metallurgical Considerations• Wire Hardness / Harder wire more damage• Bond Pad Thickness / Thinner pads more
damage• Bond Pad Hardness / Harder metal more damage• Pad Inter-layers / Hard layers can lessen
damage• Intermetallic stresses / Higher tensile stresses
more damage & controlled by post processing
PWB Materials / Processing• Glass weave / Additives / Fillers• Bonding Pad construction
• Bond Pad Thickness Control / Thinner harmful• Bond Pad and Interface Metal Hardness / Harder
harmful• Fracture Properties of Pad Under-layers /
Lowers crack initiation stress & propagation stress
• Time, Temperature, and Cooling Rates of Heat Treatments / Sources of non-uniform stresses
Thermocompression Bonding Substrate Deformation
Bonding Details
Commonly used Wire Bond Tests• Pull Test (process control)
– Destructive– Non-destructive
• Shear Test• Centrifuge• Mechanical Shock• Thermally induced stress tests• Vibration (fatigue and variable frequency)• ??
15
Common Failure Mechanisms• Intermetallic formation
(Cu oxidation / /corrosion)• Thermal stress
– Metal fatigue– Wire creep
• More to be added by team
16
Wire Bonding Optimization (Considerations)
Design Parameters Wire Bonding Impact• Pad Size Wire Diameter
(Chip & Substrate) Ball Size
• Chip & Substrate – Size Bond Distribution– Metallization Bondability
• Pad Placement Wire AngleWire Length
• Substrate Technology BondabilityReliability
17
Wire Bonding Optimization (Impact)• Improved Yield• Improved Quality / Reliability• Increased Efficiency• Reduced Design Interval
18
19
Project Driver• New substrate materials are being introduced to satisfy
industry’s desire to have halogen flame retardant free material systems
• Many people in the electronic industry would agree that due to the complexity of the Cu wire bonding process, characterizing new material systems is marginal
• There is very little agreement on the test methods and conditions which should be used for evaluating and characterizing the variety of PWB substrate technologies in the market
20
Project Proposal• Develop / identify an industry wide accepted set
of test methods for the evaluation of existing and new PWB packaging substrate technologies
• Identify test conditions that provide the necessary data for the industry to understand the critical parameters needed to characterize existing and new PWB substrate technologies
Goals of the Project• Define the PWB material property requirements as well as the
associated test conditions that deliver the data required to make design and supply chain readiness decisions
• Leverage prior investigations, carry out the necessary testing of available materials
• Analyze results• Identify inadequacies (gaps) in existing test methods and
propose solutions to eliminate the gaps• Publish a set of material guidelines for use by industry that
meets all market segment requirements• Drive test conditions and material properties into supplier
data sheets. – This may entail engaging IPC or JEDEC to revise existing industry
standards
21
Additional Input Proposed for Project• Include material properties of the molding
compound, such as: – pH values– Free ion levels– Etc. – These may not be an issue today for Au wire, but may need
to be limited and controlled for Cu wires• Correlation of bonding parameters to IC material
change (Low K, ELK & ULK)• Reliability characteristics on temperature, humidity
& bias conditions• Economics and advantages of Pd coated Cu wire
22
