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Contamination causing shorts.

Charlie

#9323

Contamination causing shorts. | 24 September, 1999

I need a memory jog. What is the term for the growth of conductive contamination after a board is in storage or field?

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C.K.

#9324

Re: Contamination causing shorts. | 24 September, 1999

| I need a memory jog. What is the term for the growth of conductive contamination after a board is in storage or field? |

I believe you're referring to "ELECTROMIGRATION."

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Chris May

#9325

Re: Contamination causing shorts. | 24 September, 1999

| I need a memory jog. What is the term for the growth of conductive contamination after a board is in storage or field? |

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Chris May

#9326

Re: Contamination causing shorts. | 24 September, 1999

| I need a memory jog. What is the term for the growth of conductive contamination after a board is in storage or field? | I believe you may mean DENDRITIC GROWTH (Dendrites).

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#9327

Re: Contamination causing shorts. | 24 September, 1999

| I need a memory jog. What is the term for the growth of conductive contamination after a board is in storage or field? | Charles: Both Chris and CK are correct, according to the SMTnet "Dictionary:"

"Dendrite. Metallic filaments growing by electromigration between two points."

Ta

Dave F

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Brian

#9328

Re: Contamination causing shorts. | 25 September, 1999

| I need a memory jog. What is the term for the growth of conductive contamination after a board is in storage or field? | There are several different mechanisms which can cause this. In reality, ionic contamination is the root cause for many of the phenomena and, in itself, can cause an ionic conductive path, although this is hardly a "short", but can cause the same effect on high-impedance circuits.

Electromigration is one phenomenon caused by ionic contamination and produces drops in surface insulation resistance. On the other hand, this is often confused with electrochemical migration, where metallic ions are removed/deposited and dendrites are one of several manifestations of this phenomenon. Another is corrosion and non-metallic corrosion products (metallic salts) can also bridge conductors, producing shorts. Another such phenomenon is CAF or conductive anodic filaments. These often grow along glass fibres in FR-4, between PTHs and is believed to be a deposit of metallic hydroxides. The exact mechanism is unknown but may have something to do with the cross-linking of epoxy molecules to the silane glass treatment with the presence of free chloride ions released by the prepolymerisation reaction with the epichlorohydrin.

Then there are whiskers. These are most commonly formed with a silver or silver-plated conductor but also pure tin can cause it. This is a time-related diffusion phenomenon caused by the metal recrystallising into a different form. In the case of silver, the crystals are acicular and can grow to several mm in length under the worst conditions, without an applied EMF. Tin is less dangerous, but most tin alloys are harmless. However, I wonder whether any research has been done on one of the lead-free solder alloys, Sn96/Ag4, which may just cause such migration.

IMHO, dendrites are possibly the second most common cause of intermittent faults (after connector problems).

Hope this helps

Brian

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-KH

#9329

Re: Contamination causing shorts. | 28 September, 1999

| | I need a memory jog. What is the term for the growth of conductive contamination after a board is in storage or field? | | | | I believe you're referring to "ELECTROMIGRATION." | Dendrites.

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Brian

#9330

Re: Contamination causing shorts. | 29 September, 1999

| I need a memory jog. What is the term for the growth of conductive contamination after a board is in storage or field? | Sorry, forgot one in my previous message: solder balls :-)

Brian

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