Wire Bonds: Good and Bad

1. Henning Leidecker Code 562 / GSFC / NASA 301-286-9180 18 January 2006 Wire Bonds: Good and Bad

2. Wire bonding: Not “romantic” but still necessary! • Some 10 trillion wire bonds are made in a year. • Most bonds work --- no one notices. • A few break, and this causes the device to fail. • Wire bonding is a robust process, and is well understood, including the occasional problems. Most problems can be isolated to a given production lot, and --- when discovered --- fixed so that subsequent lots are OK.

3. Example of ball lift (Atlantis: Oct 2000)

4. Wire bond rupture found this month in a GSFC Project (NOT Actel-related)

5. Close up of ruptured bond wire shown in previous slide.

6. Places to go to learn about bonding • George Harman Wire Bonding in Microelectronics Materials, Processes, Reliability and Yield, 2nd Edition, McGraw Hill, 1997 This is an excellent place to start for the casual observer as well as professionals in the electronic parts industry. • http://nepp.nasa.gov/wirebond/ NASA Goddard Space Flight Center Wire Bond Website: Selected Topics. One of the 'selected topics' discusses how 'shallow wire bonds' can cause apparently weak bonds.

7. GSFC's adventure with apparently weak bonds on an RTAX1000S • In Nov 2005, OLD & Code 562 sent two examples of each of the RTAX250X, the RTAX1000S, and the RTAX2000S to Hi-Rel Labs for DPAs based on GSFC S-311-M-70, MIL-STD-1580B REQ. 16.1, MIL-STD-883 Method 5009; a 'prohibited materials analysis' was added. • The pair of RTAX250X and the pair of RTAX2000S met all standards. • The first RTX250S examined did not meet the minimum bond strength: • MIL-STD-883, method 2011, para. 3.2, requires a minimum strength of 1.5 g_f, but two wires broke at 1.3g_f and at 1.4g_f respectively, each at the die heel. • The DPA was interrupted so that SEM images could be acquired. These showed that some of the die bonds have some micro-cracking at the die bond/heel interface.

8. Example of four acceptable wedge bonds in the subject RTAX1000S

9. Multiple cracks in heal region of a bond on the subject RTAX1000S: These cracks are stress-risers that reduce the load needed to break the bond. This is one example of “many”.

10. Force diagram relating 'load in wire' to the force applied by the hook --- from Harman: “Wire Bonding”

11. Expressions for using the force diagram (from Harman):

12. Image showing the two distinct lengths of the bonding wires, and their numbering, for subject device.

13. Examples of force multiplication: • Let the length of the bond wire be 'd = 5.0 mm'. • Place the hook at the middle of the wire:  = 0.5 • Lift upwards with 1.5 gf. Then the tension in the wire is... • 1.35 gf when the lift H+h is 3 mm • 2.02 gf when the lift is 2 mm • 4.04 gf when the lift is 1 mm. • Note the growth in wire tension as the wire “dress” is lowered.

14. Typical results found at Hi-Rel Labs when testing three examples of a new lot of RTAX1000S, D/C 0546: NO HEAL CRACKS & GOOD STRENGTH

15. Message of this exercise... • A single broken bond wire can destroy the functioning of a device. • The rules for wire bonding are well understood in principle. • Accidents happen, resulting in sub-standard bonds. Hence, testing is worthwhile. • When testing finds problems, the solution is usually direct and straightforward.