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SMT electronics assembly manufacturing forum.


Electroless Tin Plating

Chuck B.

#10021

Electroless Tin Plating | 20 August, 1999

I am being asked, by my vendor and Purchasing people about converting from Tin/lead pads on P.C.B's to Electroless Tin Plating. Can any one tell me what I have to change in my SMT process (paste, reflow, etc) If any? I ran one P.C.Board and I noticed the solder did not wet the whole pad. This was noticed on larger pads only. I know, how large is large, anything from 1812 package pad config. and larger.

Thanks in Advance

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Earl Moon

#10022

Re: Electroless Tin Plating | 20 August, 1999

| I am being asked, by my vendor and Purchasing people about converting from Tin/lead pads on P.C.B's to Electroless Tin Plating. | Can any one tell me what I have to change in my SMT | process (paste, reflow, etc) If any? | I ran one P.C.Board and I noticed the solder did not | wet the whole pad. This was noticed on larger pads only. | I know, how large is large, anything from 1812 package | pad config. and larger. | | Thanks in Advance | Electroless tin deposition is a "coating" process - not a plating process. As such, it does not have the advantage of being as "intimate" with the board's copper surface, and below, as electroplated chemistries and processes. This makes it a bit analog to HASL in one respect.

The last process the board's surface, before immersion or HASLing, must render it perfectly clean so specified adherence is assured. This often is not done, nor can it always be, so copper oxidation creeps into the picture early.

Also, tin oxidizes at a rapid rate, compared with tin/lead plating (especially that having been fused properly). This makes the solder termination areas non-wettable. A combination of the two defects renders solder wetting often impossible.

Tin immersion is not a recommended coating process to ensure solderability. This especially is true of "grey" types. "White" tin was discussed early on this forum.

Earl Moon

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Carol Zhang

#10023

Re: Electroless Tin Plating | 20 August, 1999

| | I am being asked, by my vendor and Purchasing people about converting from Tin/lead pads on P.C.B's to Electroless Tin Plating. | | Can any one tell me what I have to change in my SMT | | process (paste, reflow, etc) If any? | | I ran one P.C.Board and I noticed the solder did not | | wet the whole pad. This was noticed on larger pads only. | | I know, how large is large, anything from 1812 package | | pad config. and larger. | | | | Thanks in Advance | | | Electroless tin deposition is a "coating" process - not a plating process. As such, it does not have the advantage of being as "intimate" with the board's copper surface, and below, as electroplated chemistries and processes. This makes it a bit analog to HASL in one respect. | | The last process the board's surface, before immersion or HASLing, must render it perfectly clean so specified adherence is assured. This often is not done, nor can it always be, so copper oxidation creeps into the picture early. | | Also, tin oxidizes at a rapid rate, compared with tin/lead plating (especially that having been fused properly). This makes the solder termination areas non-wettable. A combination of the two defects renders solder wetting often impossible. | | Tin immersion is not a recommended coating process to ensure solderability. This especially is true of "grey" types. "White" tin was discussed early on this forum. | | Earl Moon | Earl, What is HASL? Tahnk you. Carol

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Earl Moon

#10024

Re: Electroless Tin Plating | 20 August, 1999

| | | I am being asked, by my vendor and Purchasing people about converting from Tin/lead pads on P.C.B's to Electroless Tin Plating. | | | Can any one tell me what I have to change in my SMT | | | process (paste, reflow, etc) If any? | | | I ran one P.C.Board and I noticed the solder did not | | | wet the whole pad. This was noticed on larger pads only. | | | I know, how large is large, anything from 1812 package | | | pad config. and larger. | | | | | | Thanks in Advance | | | | | Electroless tin deposition is a "coating" process - not a plating process. As such, it does not have the advantage of being as "intimate" with the board's copper surface, and below, as electroplated chemistries and processes. This makes it a bit analog to HASL in one respect. | | | | The last process the board's surface, before immersion or HASLing, must render it perfectly clean so specified adherence is assured. This often is not done, nor can it always be, so copper oxidation creeps into the picture early. | | | | Also, tin oxidizes at a rapid rate, compared with tin/lead plating (especially that having been fused properly). This makes the solder termination areas non-wettable. A combination of the two defects renders solder wetting often impossible. | | | | Tin immersion is not a recommended coating process to ensure solderability. This especially is true of "grey" types. "White" tin was discussed early on this forum. | | | | Earl Moon | | | Earl, | What is HASL? | Tahnk you. | Carol | Hot air solder leveling is HASL.

Earl

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Brian

#10025

Re: Electroless Tin Plating | 21 August, 1999

| I am being asked, by my vendor and Purchasing people about converting from Tin/lead pads on P.C.B's to Electroless Tin Plating. | Can any one tell me what I have to change in my SMT | process (paste, reflow, etc) If any? | I ran one P.C.Board and I noticed the solder did not | wet the whole pad. This was noticed on larger pads only. | I know, how large is large, anything from 1812 package | pad config. and larger. | | Thanks in Advance | Chuck

Sure it's more profitable for the vendor and he can even reduce his price somewhat, so it's attractive for the purchasing guys, BUT, as sure as God made little green apples, YOU are the one who will suffer from poor solderability and short shelf life of the PCBs. If you want an alternative to your current finish, I'd prefer bare copper with an organic solderability protection. At least, if anything goes wrong, it is easy to recover the situation.

Brian

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#10026

Re: Electroless Tin Plating | 23 August, 1999

| I am being asked, by my vendor and Purchasing people about converting from Tin/lead pads on P.C.B's to Electroless Tin Plating. | Can any one tell me what I have to change in my SMT | process (paste, reflow, etc) If any? | I ran one P.C.Board and I noticed the solder did not | wet the whole pad. This was noticed on larger pads only. | I know, how large is large, anything from 1812 package | pad config. and larger. | | Thanks in Advance | Chuck: Rather than changing your process, you should be changing your board supplier. Either you�ve got the facts crossed or your supplier is a maroon.

In recent times, I have never heard of electroless tin used as a solderability preservative on the outer surfaces of a printed circuit board. Recently, I�ve seen it used as:

� EMC / RFI shields on plastic packaging. � Inner coating of multilayers of fabs to improve adhesion between the layers. � Etch resist on fabs.

Just so that we�re dancin� an� kickin� to the same music, in electroless (electrode-less), chemical plating the deposition is carried out in a solution and is based on chemical reactions (mainly reductions) without an external source of electric current. The two different process principles are:

� Autocatalytic-plating, for nickel, copper, and their alloys. � Ion exchange-plating (very thin coatings) for tin plating of copper and aluminum.

Unlike electroplating, electroless coatings are usually very uniform with a thickness > 0.6 �m (25 micro inches) and self-limit around 5 mils depending on the material being plated and the process.

Similarly with white tin, laminates with cleaned copper are immersed in a series of tin baths without an external source of electric current to "build-up" a coating. This tin coating ranges 0.7-1.0 �m (30 to 40 micro inches).

Now here�s the rub for you. When joining 63Sn/37Pb and other high tin alloys with copper, two intermetallic compounds are formed. On the copper side is Cu3Sn and on the solder side, the relatively rough and irregular Cu6Sn5. The total thickness of the intermetallic layer is usually 0.5-0.7 �m.

Gawd!!!! That�s the same thickness as the rinky-dink electroless tin plating on your board. As we speak, the IL is punching through the plating surface and making it unsolderable. And it�s still growing as we scribe.

Here�s what you do:

� Don�t waste your time putzing around with those boards. � Get your supplier to section the board and assess the IL and plating thickness. � Go to the Hadco site and get their "boiler-plate." Use it and some of the Earl Moon specs in the SMTnet archives as the basis for your board spec.

As Brian said (in effect), the pennies your buyer is saving to get his/her bonus will cost you both dollars to repair and your bonus ... and maybe more.

my2�

Dave F

PS. If this is the gray tin plating, that Earl mentioned, you should be concerned about the long term reliability of your product (it'll be poor) after you figure-out how to solder the boards. Ta.

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