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ref: -0 tags: nickel chrome polyimide adhesion date: 10-11-2014 00:13 gmt revision:7 [6] [5] [4] [3] [2] [1] [head]

Adhesiveless copper on polyimide substrate with nickel-chromium tiecoat

  • Chrome works the best, with Nichrome lagging slightly behind. Thicker tie layers (20nm) work slightly better.
  • 17 nm Cr and 5nm NiCr both work well after gold plating
    • in aggressive cyanide solution -- without tie layer, the copper was released.
    • note how thin the layers are!
  • Surface benefits from oxygen plasma pre-treatment. (de-scum?)
  • Still not sure how to get second layer of polyimide to adhere to top layer of Cr.

Adhesion Between Polymers and Other Substances - A Review of Bonding Mechanisms, Systems and Testing

  • The adhesion between the polyimide, PMDA-ODA and metals such as copper or chromium has received considerable attention due to its importance in the microelectronics industries.
  • As mentioned, the PMDA-ODA is normally deposited from solution as the polyamic acid and cured in-situ to the imide form.
  • Adhesion of the polyimide deposited on a metal is therefore a different problem than adhesion of a metal deposited on the cured polyimide.
  • The former situation (polyimide on metal) tends to give stronger adhesion than the latter (metal on polyimide) but there can be problems of metal, particularly copper, dissolution.
  • Great! (is this a reliable source?)
  • The interaction between the metals and the polyimide has been studied in great detail using x-ray photoelectron spectroscopy (XPS) and other surface analysis techniques but there is not complete agreement on the form of the interaction.
    • It is clear that strong interaction and electron transfer occurs when the metal is deposited from vapour onto the polyimide.
    • When the polyamic acid is deposited on the metal and cured then reaction occurs between the acid and the metal.
  • The strong interface formed between chromium and the polyimide is clearly a result of the strong chemical interaction but there is still considerable interest in making it more resistant to water and oxidation.

High-Performance Polymers (book) Guy Rabilloud (via google books.)

  • Order of metals by increasing adhesion:
    • Cu, Pd, Ni, V, Cr, Nb, Ti [140]
  • The adhesion between chromium and polyimide is degraded sharply as the interface is exposed to temperature-humidity stressing (85C, 81% RH [612]
  • Polyimide-polyimide self-adhesion strongly benefits from partial cure of the first layer (which is not possible with lithographic processes, TMAH etches uncured film). Plasma and adhesion treatments would likely help, due to molecular tangling (?). Presumably VM-651 helps. We'll cross that bridge when we get to it.
  • PMDA-PPD or PMDA-PDA is perhaps the most rigid of all the polyimides, but due to the extremely hydrophillic nature of PMDA & associated electron affinity of the dianhydride ( E aE_a ), and the fact that it tends to crystalize & not be tough/plastic, it's infrequently used.

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ref: -0 tags: wirebonding finishes gold nickel palladium electroless electrolytic date: 09-21-2014 02:53 gmt revision:3 [2] [1] [0] [head]

Why palladium?

To prevent black nickel: http://tayloredge.com/reference/Electronics/PWB/BlackPad_ITRI_Round1.PD

Introduction The use of electroless nickel / immersion gold (E.Ni/I.Au) as a circuit board finish has grown significantly in the last few years. It provides a flat board finish, is very solderable, provides a precious metal contact surface and the nickel strengthens the plated holes. However, as the usage of E.Ni/I.Au increased, a problem was found on BGA (Ball Grid Array) components. An open or fractured solder joint sometimes appears after board assembly on the occasional BGA pad. The solder had wet and dissolved the gold and formed a weak intermetallic bond to the nickel. This weak bond to the nickel readily fractures under stress or shock, leaving an open circuit. The incidence of this problem appears to be very sporadic and a low ppm level problem, but it is very unpredictable. A BGA solder joint cannot be touched-up without the component being removed. After the BGA component is removed, a black pad is observed at the affected pad site. This black pad is not readily solderable, but it can be repaired.

From: http://www.smtnet.com/Forums/index.cfm?fuseaction=view_thread&Thread_ID=4430

You don't have enough gold. Your 2uin is too porous and is allowing the nickel to corrode. Prove that this by hand soldering to these pads with a more active flux, like a water soluble solder paste, than you are using.

You must have at least 3uin of immersion gold. Seriously consider >5uin.

Your nickel thickness is fine. Although if you wanted to trade costs, consider giving-up nickel to 150uin thickness, while increasing the gold thickness. Gold over electroless nickel creates brittle joints because of phosphorous in the nickel plating bath. The phosphorous migrates into the over-plating. Electrolytic nickel and gold plating should not be a problem.

If you stay with the electroless nickel, keep the phosphorous at a mid [7 - 9%] level. Just as important, don't let the immersion gold get too aggressive. The immersion gold works by corroding the nickel. If it is too aggressive it takes away the nickel and leave phosphorous behind. This makes it look like the phosphorous level is too high in the nickel bath.

Gold purity is very important for any type of wire bonding process. For aluminum wedge bonding, gold should have a purity of 99. 99% [no thalium] and the nickel becomes critical. No contaminates and the nickel wants to be plated a soft as possible. This requires good control of Ph and plating chemicals in the nickel-plating bath.

Harman "Wire Bonding In Microelectronics" McGraw-Hill is a good resource for troubleshooting wire bonding. I reviewed it in the SMTnet Newsletter a couple of months ago.

That said, electrolytic nickel + electrolytic gold does work well -- perhaps even better than ENEPIG: