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silver finishes on pcbs

jacqueline

#2219

silver finishes on pcbs | 13 December, 2000

Hi there,

Could someone please tell me diferences and possible benefits that are found when using silver as a pcb finish as opposed to HASL. I've already tried the archives without much luck.

ta much, jack

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#2220

Re: silver finishes on pcbs | 13 December, 2000

Hi Jack,

I once tried this finish ( without changing any process parameters ) and encountered wetting problems even in the first reflow process. It was thought as cheaper alternative to Ni/Au we use for planarity reasons. Heard of the same problems others had. It might be that things are different today than 4 years ago. At least your supplier should proof that whatever you want to do with the PCB will work or tell you which process adjustments are necessary.

What I do with new material is always a testrun to see if and how it works. BTW, the supplier doesn�t offer this finish anymore here.

Others may have different oppinions about that.

Wolfgang

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#2221

Re: silver finishes on pcbs | 13 December, 2000

Silver finish???? What kind ??? electroplated or immersion??? [anyone giving hints??]

ELECTROPLATED

Silver should never be electroplated onto copper, because it diffuses into the copper at a blistering rate and then solderability goes into the dumper. [Silver migrates into copper at a rate 1,000 times greater than tin according to Tautscher.] You can watch a 1 um silver plating on copper disappear in less than 24 hours. Seriously consider using a nickel buffer against this diffusion.

Some people talk about plating silver to a 5um thickness to overcome this diffusion of silver into the copper. This thickness of silver does remain silver that�s nice and silvery, but it�s: * Expensive * Possible that this is a poor approach for a solderable surface stand point. Silver-tin intermetallics are long and brittle. When the silver content approaches 5% lead-tin solder connections become very unreliable.

Fresh silver is very solderable. Sulfur in the air makes silver less solderable and reacts with silver to form silver sulfide that could creep and cause shorts to adjacent leads. * Proper protective packaging could prevent this [I have no experience in trying] deterioration. I do know that cardboard contains beaucoup sulfur. * OSP on the silver may help.[Again, no experience.]

IMMERSION

Did someone say OSP? On OSP soaked silver things ... No comments beyond those in => Immersion Silver PCB Surface Plating - Dennis VanBuren 14:21:24 02/01/2000 [Wudja mean "not much luck" searching the archives? What is this haggis???][Is that like saying "what is this chopped liver??"]

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PeteB

#2222

Re: silver finishes on pcbs | 14 December, 2000

Jacqueline,

Asuming you mean immersion silver:

Advantages - Very flat finish compared to HASL, more easily solderable than bare copper with OSP and more process tolerant than this in our experience, reasonable shelf life, cheaper than immersion gold and doesn't require nasty chamicals (immersion gold)or high temperature excursions (HASL)in the PCB.

Disadvantages - Not very widely available, requires careful handling to avoid damaging OSP coating, mind you, handling PCB's by their solderable surfaces is not good practice anyway!!

We have been using this finish on some of our finer pitch assemblies for about a year now (Alpha Level). The majority of these are reflow and wave solder or multiple reflow processes. We are using the same process setups as for equivalent HASL boards and have not seen any real difference in wetting performance and it is better than immersion gold.

Regarding the process time limitations as mentioned by Wolfgang, a good production control regime means that partly processed PCB's should not hang around on the shop floor for too long anyway. Typically ours have no more than a max. of 3 days between all the various processes.

Storage time for immersion silver - we have a max. shelf life limitation for stock of 6 months, our supplier guarantees solderability for 12 months minimum.

All in all we rather like it over the alternatives.

regards,

Pete B.

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JohnW

#2223

Re: silver finishes on pcbs - what about the wave? | 15 December, 2000

Dave...think you've gone off on one again..., but if you want th secret family recipe for Haggis let me know....actually catch one 1st then I'll tell you how to cook it!

Ok so we've got mixed opinion's on the silver then, and mostly based round SMD. What about over wave? how many of you are running this over wave as no clean and what fluxes are you using and @ what solid's content?...

John

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Pete B

#2224

Re: silver finishes on pcbs - what about the wave? | 15 December, 2000

John,

We regularly wave solder electroless silver finished boards (Alpha Level)with mixed SMD and through hole parts and do not have any problems.

We have changed a number on PCB's from HASL to electroless silver, mainly for the benefits at SMD when placing fine pitch flatpacks. We were wary at first but find the settings do not need alteration from the original HASL setup. Excellent hole fill and wetting including on heavy multilayers.

For your information we use a water based VOC free flux (Hi- grade 1075 - EX30 supplied to us in the UK by BLT circuit services). Solids contant of this flux is 2%

With regard to bath contamination from silver dissolution we have not seen any significant difference between this and gold contamination if we were to use this instead.

Hope this info is useful.

Regards,

Pete B

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Raul

#23478

Immersion Silver - Sulphur | 21 February, 2003

It is a common knowledge that sulphur and immersion silver finish on a PCB do not go together very well. We are seeing peaks of sulphur on some of our assemblies (with silver finish) and as expected we experience severe difficulties with processing these such as discolored pads, non-wetting etc.

Is there something in the PCB assembly line, that can induce sulphur on the PCBs?

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#23484

Immersion Silver - Sulphur | 23 February, 2003

Sources of sulphur in PCB fabrication are: * Some locales have high H2S concentrations in the air. * Metal etching steps that use sulfuric acid or a sulfonated buffering compound may result in sulfate residues. * Sulfates can come from contact with normal brown papers and cardboards, since sulfuric acid is used in their manufacture. * Tap water rinsing during board fabrication is the most common source of sulfates. Almost all tap water has sulfate and hard water areas (e.g. the Dakotas) have high levels of sulfate.

Sources of sulphur in PCA assembly are: * Some locales have high H2S concentrations in the air. * Sulphur can be used in the elastomer cure process and can out-gass, especially if the cure is not complete. * If using paper separators or wrapping ensure you specifiy lint free and sulphur free. * Sulfates can come from contact with normal brown papers and cardboards, since sulfuric acid is used in their manufacture. * Smoking most certainly gives off "solids" in the exhaled smoke, and this is prone to settling on whatever it contacts. * Many real rubber and plastics degrade slightly and release sulfuric acid. (Paper does this also, look at any old newspaper. The yellow color is from the acid.) There are also might be some unbound sulfur in the rubber, left from the vulcanization process.

Finally, Mac Dermids and Alpha give recommendations for handling and storing imm silver boards. This could be a good starting point for your and your supplier's handling practices.

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