Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

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imm. silver



imm. silver | 29 August, 2002

I have seen several Imm Ag pad on the assembled board which have a copper colorleaching through the silver plating This happened on the pad which was not deposited with paste during the assembly process.

Don't know if it is a problem, but we only had these assembled boards a month and it appears they are already "oxidizing/leaching" whatever the correct term is. By the way, the boards have just been sitting around at no extreme temp or humidity cycles.

Guys, would you know anything about the immersion silver where all pads need to be reflowed?

Pls help.

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imm. silver | 29 August, 2002

Often assemblers leave areas unsoldered for future component additions, modifications, and test points. On imm silver boards, these unsoldered areas become a real bear to solder/test later in life. One way to avoid that problem is deposit some solder on those areas during initial assembly.

Other than not being able to solder/test these pads, what's the problem?

What is Nanya [Zat NTC]?

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imm. silver | 30 August, 2002

Hi Dave,

Why does it become " a bear" if u leave it unsoldered ?? Does it oxidise or something ?? Question (1): Does Ag imm. PCB need to use Solder paste with AG ?? If yes, why.

Question (2): Does Ag imm pcb have poorer surface finishes than HASL or Entek coated ones ??

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imm. silver | 30 August, 2002

Possibly this is tarnish? Silver finish requires a bit more care in handling or packaging - to avoid contact with sulphur which may cause tarnish. Some cardboard boxes gas out enough sulphur to cause a problem. Possibly look at your method of storage.

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imm. silver | 30 August, 2002

Imm silver holds-up to multiple thermal cycles very well. The things, we guess, are happening to make imm silver tough to solder after heating and sitting are: * Oxidation of the silver. * Reduction of the co-deposited organic anti-tarnish [used in some coating types] thickness during the heat cycles. * Formation of Ag intermetallics

The people at Alpha or MacDermid could give you better answers. Please let us know what they say.

Q1: Does Ag imm. PCB need to use solder paste with Ag?? If yes, why? A1: Imm silver does not require silver paste. [Although we run at lot of 2%, it has nothing to do with imm silver pads.] The amount of silver in a solder connection as the result of imm silver on the pad is very small regardless of the type of paste.

Q2: Does Ag imm pcb have poorer surface finishes than HASL or Entek coated ones?? A2: We think imm silver is superior to both HASL and OSP for modern boards.

TABLE 1 - PROPERTY COMPARISON OF ALTERNATIVE COPPER PROTECTIVE COATINGS ||ALTERNATIVE COATINGS [from Alpha metals] ||Hot Air||Rosin/Resin||||Amidazole/||Immersion||Electroless||Immersion Property of Copper Coating||Solder Leveled||Coating||Benzotriazole||Benzimidazole||Bismuth||Nickel/Gold||Silver (Alpha Level) Lowest Cost = 10; Highest = 1 Cost of PCB Fabrication||5||8||10||7||7||1||5 Most Pin Testable = 10; Least Pin Testable = 1 Pin Testability of Bare Board||10||1||3||4||10||10||10 Longest Shelf Life = 10, Shortest Shelf Life = 1 Shelf Life (time between board fabrication and assembly)||9||6||3||6||7||9||9 Least Contamination = 10; Most Contamination = 1 Ionic Cleanliness and Polyglygol-free Bare Boards||1||5||10||10||10||10||10 Flat Pads = 10; Non-Planar Pads = 1 Flat, Planar Pads for Successful Placement of Fine Pitch SMD (< 16 mil)||1||10||10||10||10||10||10 Not Easy To Remove = 10; Easy To Remove = 1 Risk of Removing Protective Coating When Cleaning Misprinted Boards||10||2||2||2||10||10||10 Highly Solderable = 10; Least Solderable = 1 Solderability at Wave Solder (no prior reflows)||10||6||6||6||7||8||9 Highly Solderable = 10; Least Solderable = 1 Solderability at Wave Solder (after 2-3 reflows) and using a no-clean flux||10||1||2||5||7||8||9 Easily Wire Bonded = 10; Can't Wire Bond To = 1 Ability of Surface to be Wire Bonded||1||1||1||1||1||10||9

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imm. silver | 8 September, 2002

My specialty. Your right, Most corrugated contains S02 and the pallet is usually out gassing formic acid as well. Not a great environment for silver.

Original press release follows:

Reactive Polymers At AT&T Bell Labs, John P. Franey has developed Intercept TM reactive polymer system incorporating scavenger molecules to neutralize corrosive gases passing through a polymer. As gases migrate through semi-permeable films and resins, they react with "a copper-like material that is bound up in the polymer".

According to an item in Science News [41(20): 321-336 May 16, 1992]

"...After spending five years in a reactive polymer bag, a silver spoon looked as if it had been polished that day, while a similar spoon not bagged for that time was black with tarnish.... The polymer darkens as pollutants exhaust its scavenging ability, but Bell Labs' studies show that a bag no thicker than a plastic garbage bag would last about 30 years before needing to be replaced."

I suggest you place the PWB's in a Static/ Corrosion Intercept poly bag and if shipping use an Intercept polymer base sheet over the wood pallet followed by Corrosion Intecept stretch wrap. This will stop the tarnish cold in any shipping or storage environment.

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