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Wire bond Concern on Actel Devices with Au wire

Wire bond Concern on Actel Devices with Au wire. Raymond Kuang Manager, Packaging Engineering May 08, 2002. Background. An Actel device RT54SX16 with date code 9937 was found by GSFC having the following: Strength of some wires pulled were below the 2.5 Grams Mil-std minimum limit

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Wire bond Concern on Actel Devices with Au wire

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  1. Wire bond Concern on Actel Devices with Au wire Raymond Kuang Manager, Packaging Engineering May 08, 2002

  2. Background • An Actel device RT54SX16 with date code 9937 was found by GSFC having the following: • Strength of some wires pulled were below the 2.5 Grams Mil-std minimum limit • Parts exhibited intermetallic halo at ball periphery. • Watermelon stripes were suggested to be contamination. • GSFC inspected additional samples taken from 11 Actel lots bonded with Au wires and result as follows: • All samples exhibited intermetallic halo around the ball periphery. • Except on RT54SX16 with date code 9937, all sample parts passed the required minimum wire pull test limits. • The bonding quality issue on RT54SX16 with date code 9937 was discovered in year 2000, resulting to PCN to customers with affected lots in July, 2000.

  3. Ceramic Capillary Au wire Failure Analysis – Low Wire Pull Strength on 9937 Lot This issue was due to the use of insufficient bonding power and force that resulted in immature welds consisted of cluster of isolated micro-welds. Since isolated micro-welds offers more grain surfaces, and therefore more vacancies defects, a rapid intermetallic diffusion characterized by vacancies defects concentration (voids) on Al-Au interface had occurred. The use of insufficient bonding force and power is evident on the shape of the ball – see photos below. Al pad Cross section image of RT54SX16 (9937 date code lot) Ball bonding on Pad

  4. Uniform intermetallic thickness Uneven intermetallic thickness Critical Factor for Reliable Au Ball Bond The only way to prevent rapid intermetallic formation and growth from happening is to assemble the part with very high quality wire bonds which exhibits a uniform metallurgical formation Figure 2 Poor quality bond with Non-uniform Intermetallic thickness Figure 1 High quality bond with uniform Intermetallic thickness A bond with uniform intermetallic formation will have stronger bond strength than a bond with spotty intermetallic formation. Figure 1 above shows an example of an ideal bond while Figure 2 shows a spotty bond that resulted to formation of voids.

  5. Actel Corrective Actions • Equipment Optimization:Upgraded bonding machines with ‘Ultra Sonic Generation (USG) delay’ and ‘Gradual Power Application’ features. These allow for application of sufficient bonding force and power in a gradual schedule to avoid damaging pad metallization. • Plasma Clean Prior to Wire bond:Helps to expose a fresh, unoxidized bonding surface in order to achieve a stronger bond interface • Units must be bonded within 3 hours after plasma clean. • These enhancements are essential to ensure a uniform Au-Al intermetallic interface that is strong and stable being created during the wire bond process.

  6. Bond pull Quality Analysis RTSX Wire pull (1.0 mil wire) Post Burn-In 100% wire pull Assembly Std Group B

  7. Bond pull Quality Analysis RTSXS Wire pull (1.0 mil wire) Break mode of minimum reading from Std. Group B and Post Burn-in 100% wire pull were “wire break at wire span”.

  8. Actel Preventive Actions • Preventive Actions with Wire Bond Process Monitor --- Wire Pull Testing • Enforcing extensive wire pull testing during assembly • To detect any sign of process or material issue that may affect bonding quality. • Require a minimum of 2 set up units prior to start of production • Production won’t start until set up units have passed pull test requirement. • In-Process Monitor • E-flow Devices : 2 additional units must go through bond pull testing in every 2 hours of production. • A minimum of 4 units per lot must be wire pulled if bonding process is completed within 2 hours. • 100% of the wires must be pulled in each sample unit. • Class B devices : 2 additional units must be wire pulled at the completion of the production lot. • 50 of the wires must be pulled from each sample unit.

  9. Actel Preventive Actions ….. Continue • Preventive Actions with Wire Bond Process Monitor --- cont. • Wire Pull Process Control limit • A guard-banded minimum allowable bond pull strength during set-up and in-process monitor is 4.5 grams. • Pre-cap Source Inspection (CSI) is required for all E-flow production lots by Actel. • Inspector will witness wire pull of 50% of the wires on 2 units randomly selected from production lot. • Disposition of Non-Conforming Lots • All production lots fail to meet minimum bond strength limit will be put-on hold for disposition from Actel • Actel will evaluate if the failure modes are true or equipment and/or operator errors • For equipment and/or operator errors, additional units must be tested and no failures allowed • Failed lots will be down graded to commercial or scrapped.

  10. Actel Preventive Actions ….. Continue • Preventive Actions with Wire Bond Process Monitor --- cont • Post burn-in wire pull monitor • 100% wire pull is perform on 2 units of the group B samples. • Tracking wire pull data on assembly set-up and group B testing on a routine base.

  11. Wire Bond Reliability Study Parts from known bad lot (9937 d/c) and good lots were subjected to Temperature Cycle Testing (TCT) for 1,000 cycles from -65°C to 150°C per Mil-Std-883, TM1010, Condition C. Wire pull test data conducted before and after TCT are compared as bellow: • The above result shows no significant degradation of bond pull strength after 1,000 Temperature Cycles. • No ball lifting exhibited on units from known good lots.

  12. Uniform intermetallic thickness Wire Bond Reliability Study ….. Continue • Wire pull test data on one unit exposed to 1,000 hours of life test at 150oC are as follows: • Post Lid SealPost 1K hours @150°C • Min. Strength: 5.8 4.8 • Max. Strength: 8.1 6.6 • Avg. Strength: 6.72 5.78 • Sample size: 100 235 • No significant degradation of bond pull strength, and no ball lifting observed. • Picture on the right is taken after 1K hours life test.

  13. Wire Bond Reliability Study ….. Continue • Wire pull test data on one unit exposed to 2,000 hours of life test at 150oC are as follows: • Post lid SealPost 2K hours @150°C • Min. Strength: 5.2 3.4 • Max. Strength: 12.7 9.5 • Avg. Strength: 6.83 7.28 • Sample size: 88 117 • Minimum reading is way above the mil-std 883 minimum bond strength requirement after exposure of unit to 150°C.

  14. Intermetallic mass Engineering Analysis – Intermetallic Mass (Halo) at Ball Edge Intermetallic mass Intermetallic mass RT54SX16 / 9937 RT54SX32S / 0049 RT54SX32S / 0049 Photo taken by GSFC ( No 1000 hrs burn-in test ) Photo taken by Actel after 1000 hrs exposure to 150°C. Intermetallic mass at ball edge RT54SX16 / 9937 RT54SX32S / 0049 • Intermetallic mass can be seen also on units with good bond quality. • Presence of intermetallic mass around the bond edge is normal and is not an indication of poor bond quality.

  15. This area will be covered depending on the capillary impression. Active contact area of Pad and Ball Engineering Analysis – Intermetallic Mass (Halo) at Ball Edge .. Cont. Ceramic Capillary Footprint of X-Y displacement of Au ball Active contact area of Pad and Ball

  16. Engineering Analysis – Intermetallic Mass (Halo) at Ball Edge .. Cont. • Formation of Intermetallic Mass (Halo) at Ball Edge: • The intermetallic mass at the edge of the ball is a result of displaced intermetallic footprint by the scrubbing motion on both X and Y directions during the initial stage of bonding. • The footprint of this Au-Al intermetallic mass will grow when the device is exposed to heat during the lid seal process. • The size of the exposed intermetallic mass around the edge of the ball is also dependent to the final ball shape. If the ball is squashed, it may cover all intermetallic mass at the ball edge. • Bonding force and power are less on area near the ball edge • than at the bond center. As a result, void is normal to occur on area near the ball edge. This void, however, will not propagate toward the • bond center and degrade the bond strength.

  17. Engineering Analysis – Dark (Watermelon) Stripes on Ball Surface • EDX analysis show no difference on the composition of dark and light spot at the ball surface. Light spot Dark spot

  18. Engineering Analysis – Dark (Watermelon) Stripes on Ball Surface … Cont. Light spot EDX analysis at 5KeV Dark spot EDX analysis at 5KeV

  19. Engineering Analysis – Dark (Watermelon) Stripes on Ball Surface … Cont. • It is suggested to be an effect of reaction between the doping materials within the wire and the heat energy of the Electronic Flame Off (EFO) spark generated during free air ball formation. • Wire Composition Analysis (Au wire vendor: American Fine Wire) • 1998 –1999Currently • Au 99.99% 99.99% • Ag 19 ppm 20 ppm • Be 6 ppm 5.2 ppm • Ca 1.5 ppm --- • Fe --- 0.8 ppm • The EDX analysis shows no evidence of organic contamination.

  20. Summary: • The affected lots, assembled in 1999, with weak bond strength and formation of voids within the Al-Au intermetallic interface were due to insufficient power and force at the wire bond process. • The intermetallic mass (Halo) around the edge of the ball is typical. • The dark stripe at the ball surface is not an indication for contamination • based on EDX analysis. • Au-Al intermetallic formation will always exist in Au wire bonded devices. • Optimized bonding process to achieve uniform intermetallic formation and high bond strength is essential to maintain device reliability. • Actel maintains tight process control and stringent wire pull monitors to ensure bond reliability. • Thanks !

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