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The Plating Forum: Wire Bonding to ENIG

The IPC-4552 ENIG specification was written in 2002, but the committee is currently in the process of updating and revising the document. The thickness of the immersion gold layer is being revised with the intent of reducing the minimum thickness from 2.0 µin to 1.6 µin. To do this, the committee undertook a series of studies to find out if this reduction on gold thickness is possible and if it could have an adverse effect on performance of the deposit.

One attribute in need of investigation/verification is wire bonding to the ENIG surface. The question is whether or not the reduced gold thickness affects the wire bondability of the surface finish. In addition, no wire bonding testing was performed by the committee when the original specification was written in 2002.

This time around, the committee designed a test vehicle to be used. In addition to wire bonding, the testing included solderability testing (wetting balance and spread test), contact resistance, and hole fill.

The test vehicle was plated with approximately six microns of electroless nickel with three different thicknesses of immersion gold. Samples from each of the three levels of gold thickness were submitted for wire bond testing. An additional sample with electroless nickel/electroless palladium with no gold was included for wire bond testing. The samples were not stressed or preconditioned prior to wire bonding, but the samples were approximately six months old and had not been stored in an “open” condition that allowed for normal atmospheric exposure.

The committee approached Mike McKeown, senior business development manager at Hesse Mechatronics, to run the desired testing and to lend his expertise in interpreting the results; the company also volunteered its services to the IPC committee. The contents of this report are the work of Dave Trawczynski, senior applications engineer at Hesse Mechatronics.

The table below shows the average of the immersion gold XRF thickness readings in micro-inches of the test vehicle samples, as well as the palladium thickness for the Ni/Pd sample, that were submitted; Figure  1 shows the testing sight in the TV.

Figure 1: Coupons tested for wire bonding.

Table 1: Thickness of precious metal on electroless Ni.

Three types of wires were tested:

• Aluminum wire 1.0 mil;
• Aluminum wire 10.0 mil; and
• Copper wire 1.0 mil.

Set-up and Results

Aluminum wire 1.0 mil

Equipment:

• Hesse Mechatronics Inc. Bondjet BJ820 Thin Wire Wedge Bonder
• Bond head: 45° bond head, 100 kHz
• Wedge: Deweyl  MCLOE-1/16-1”-45-C-2525-M
• Wire: Coining (SPM) 1% SiAl .001” 1-4% EL, 14–16 gr BL

Bonding Conditions:

• Starting Height: 800 µm                
• Touchdown Area: 100 µm                
• Touchdown Velocity: 2,000 µm/s          
• Touchdown Force: 20 cN                    
• Stop after deformation: 30%                                                       
• Max welding time: 50 ms                   
• Ultrasonic Bondforce: 20 cN                    
• Ultrasonic Duration: 50 ms                   
• Ramp: 10 ms   

Aluminum wire bonding 10 mil

Equipment:

• Hesse Mechatronics Inc. Bondjet BJ939 Heavy Wire Wedge Bonder
•  Bond head: 60 kHz
•  Wedge: Deweyl  CSIVG-10-1/8-2.50-70-MP
•  Wire: Heraeus H11 .010” Al, 10-17% EL; 200-300 gr BL

Bonding conditions:

• Starting Height: 5,000 um              
•  Touchdown Area: 200 um                
•  Touchdown Velocity: 10,000 um/s        
•  Touchdown Force: 400 cN                  
•  Stop after deformation: 80%                                                       
•  Max welding time: 95 ms                   
•  Ultrasonic Bondforce: 400 cN                  
•  Ultrasonic Duration: 20 ms                   
•  Ramp: 0 ms                      

Copper wire bonding 1.0 mil

Equipment:

• Hesse Mechatronics Inc. Bondjet BJ820 Thin Wire Wedge Bonder
• Bond head: 45° bond head, 100 kHz
• Wedge: Deweyl  MCLOE-1/16-1”-45-C/CG-2020-M
• Wire: Heraeus .001” Cu MaxSoft2  H.A.D.

Bonding Conditions:

• Starting Height: 800 um                
• Touchdown Area: 100 um                
• Touchdown Velocity: 2,000 um/s          
• Touchdown Force: 35 cN                    
• Stop after deformation: 35%                                                       
•  Max welding time: 75 ms                   
•  Ultrasonic Bondforce: 25 cN                    
•  Ultrasonic Duration: 75 ms                   
•  Ramp: 0 ms

Conclusions

• As the data indicates, aluminum wire at 1.0 mil and 10 mils as well as 1.0 mil copper wire were successfully bonded to the surfaces tested.
• It is also fair to conclude that the thickness of the immersion gold is not a critical parameter for the bonding.
• As little as 1.2 µin of gold successfully preserved the electroless nickel surface for bonding.
• The bonding tested showed excellent pull strengths for the electroless nickel electroless palladium sample that was included in the study.