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

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


ENiG or IAg? Which is better?

Views: 9631

Folks, what is the industry consensus? Which is the more po... - Mar 06, 2008 by Samir Nagheenanajar  

#53916

ENiG or IAg? Which is better? | 6 March, 2008

Folks, what is the industry consensus? Which is the more popular lead-free, flat finish these days?

Here's what I know from my experience.

IAg:

1. Quality of finish is dependent on supplier

2. Can NOT be reworked. It is known that reworking that by reworking IAg, you will not achieve intermetallic with the base copper.

3. Not a standard process at alot of PCB houses. PCB houses tend to subcontract IAg plating.

4. Known to tarnish - black pad.

From my experience with ENiG, OTOH, I've seen black pad, but the finish was still "wettable." Other than that, I've heard of no major issues.

reply »

#53917

ENiG or IAg? Which is better? | 6 March, 2008

Black pad with IAg??? Tarnish is NOT black pad. Tarnish is usually just some silver sulfide on the surface and can also usually be soldered through, by using a little additional, or slightly more agressive, flux, and still make a good quality solder joint to the copper under the silver. Black pad is an ENIG feature, not an ImAG! you are soldering to the thin gold layer on top of the nickel - the nickel has oxidised underneath. There is no real bond strength between the gold solder joint and the nickel layer underneath, which is what you are SUPPOSED to be soldering to. That is why components can be just flicked or knocked off the board. I would take ImAg over ENIG any day.

reply »

#53920

ENiG or IAg? Which is better? | 6 March, 2008

Thanks, I stand corrected on the tarnish definition.

Thanks for the input too. That's exactly what I wanted.

reply »

#53921

ENiG or IAg? Which is better? | 6 March, 2008

Long, yellow, ugly toe nails will have the same affect on your boards.

reply »

#53925

ENiG or IAg? Which is better? | 6 March, 2008

KRIKIES Chunks me-lady. Yellow toe-nails with tree rosin will definitely sodder that there immersion silver.

Sounds like i have met me match. Who is this fella with the toe-nails?

reply »

#53926

ENiG or IAg? Which is better? | 6 March, 2008

Some guy who things he's cool cause he drives a Vette whilst his collar is up.

reply »

#53927

ENiG or IAg? Which is better? | 6 March, 2008

I have not had problems with ENiG panels, but they were made state side. We had Trace Labs cross-section them and found no problems.

Whats this about yellow nails? Mine are full of grease from constantly having to fix the Jeep. I would try to pop my collar, but it would be full of grease stains.

reply »

#53929

ENiG or IAg? Which is better? | 6 March, 2008

LISTEN forget about all this stuff with corviettes and jeeps. ive driven a Gremlin since '77.

reply »

#53930

ENiG or IAg? Which is better? | 6 March, 2008

I guess you are right, they both need some loven

reply »

#53931

ENiG or IAg? Which is better? | 6 March, 2008

Sounds like bad mechanical skillz. Thank gawd you don't own a 2002 Ford Explorer!

reply »

#53932

ENiG or IAg? Which is better? | 6 March, 2008

I hear that. A 2002? You poor b-stard! That's like having cold solder joints! (Gotta keep it related)

reply »

#53933

ENiG or IAg? Which is better? | 6 March, 2008

Oh my, a Ford? Did you lose a bet?

reply »

#54030

ENiG or IAg? Which is better? | 14 March, 2008

Samir, To correct some misconceptions on the answers to your question, and "what you know from experience": With both ENIG (Electroless Nickel/Immersion Gold-IAu) and IAg (immersion silver), soldering does not take place to the gold or the silver. When the molten solder contacts the gold in the case of the ENIG finish, the 3-5 uinches of gold is immediately and completely dissolved into the solder, and the molten solder then forms a bond to the nickel underplating. A small amount of nickel dissolves into the solder to form the Intermetallic Formation (IMF).

Likewise, when molten solder comes in contact with immersion silver (IAg), the 10-18 uinches of silver immediately dissolves into the molten solder, and then a small amount of the copper underneath the silver dissolves into the solder also, again forming the IMF. One does not solder to either the gold or the silver. Those noble metals are there simply to prevent the nickel from oxidizing and the copper from oxidizing. The process of ENIG plating is much more complex and harder to control than IAg, and to make a long story short, a certain amount of phosphorus (P) is required for the ENIG plating process. If there is too much P, a nickel oxide layer will form underneath the gold, on top of the nickel. This is basically the cause of the "Black Pad". This layer of oxidized nickel poses problems in that it prevents the solder from dissolving a certain amount of nickel. Standard solder fluxes have no effect on nickel oxides, they are not formulated for that purpose.

There are no black pad issues related to immersion silver finish. Copper's rate of dissolution into solder is much faster than nickel's rate of dissolution. So, when soldering to IAg finish (immersion silver over copper) you are forming an IMF with copper, and copper readily dissolves into the solder. Thus, it is far easier to form a good IMF with copper than it is with nickel. Nickel's rate of dissolution into the molten solder is much slower than that of copper. Thus, when you are soldering to ENIG, you need slightly higher temperatures and for a longer time in order to dissolve enough nickel to form a good IMF with the molten solder. For this reason, many prefer to solder to immersion silver finishes. There are also other issues with solder to ENIG I have not discussed here.

There is no reason why one cannot rework IAg solder joints. Your statement that it cannot be done is completely untrue. Solder rework is performed on both of these finishes by the thousands every single day, and if done properly there are no reliability issues with a reworked solder joint versus a virgin sj on either finish. Finishes must be chosen during the design stage for many more reasons than just solderability. Careful consideration must be given to many other field performance issues. I have written many papers on this subject that I can share with you should you desire. If you change the finish, it should only be done after a careful qualification.

reply »

#54043

ENiG or IAg? Which is better? | 14 March, 2008

Dean, thanks for schooling me on this.

I talked to an industry collegue whom I met at APEX, and it was he who had told me of the IAg "rework issue." Basically, he told me that they had field failures on QFPs that were a direct result of trying to "replate" IAg. I'm not sure if this information is accurate, 'cause from what you're saying above, the Ag merely acts as an oxide shield, and you're not relying on it for IMF. Perhaps the silver layer became too thick, and inhibiting SnCu intermetallic??

Personally, I haven't had issues with either finish aside from the fact that each finish looks dull and grainy, and is without the pretty fillets that you'd see with tradition Sn-Pb HASL.

reply »

#54050

ENiG or IAg? Which is better? | 15 March, 2008

Unlike other solderability protection, immersion coatings [eg, ImAg, ImSn, OSP, etc] can be reworked by the board fabricator prior to assembly operations.

Reasons for selecting immersion are: * Until a supplier can demonstrate that they can turn black pad on /off, not just prevent it, we'll take immersion, because it's controllable. * Immersion is reworkable, ENIG is not * ENIG 100, ISn 78, DIG [direct immersion gold on copper] 60, IAg 43 [Saturn Electronics] * With immersion soldering occurs at the copper surface, whereas with ENIG soldering occurs at the nickel overplate surface.

reply »

#54051

ENiG or IAg? Which is better? | 15 March, 2008

As a rework house to contract manufacturers, a majority of the BGA rework we receive is failing due to Black Pad issues. Recently we have been taking in a lot of consumer products that have parts just falling off the lead free boards that are assembled in China due to poor reflow. Just a matter of time before things start to fall out of the sky I guess. Someone should notify the US Govt. of this before they start to send any more contracts overseas. At least that's my spin on it.

reply »

#54057

ENiG or IAg? Which is better? | 16 March, 2008

Dennis: What do you do when you receive a black pad board? How do you rework it?

reply »

PCB Buffers

Boundary Scan