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Immersion White Tin Process for PCB Finish

Pat Pepper

#7981

Immersion White Tin Process for PCB Finish | 4 January, 2000

Hi everyone:

Our company is trying out immersion white tin for fine pitch assembly with what I would call mixed results. Reflow solder looks very good, but wave solder looks somewhat grainy (this may not be a problem in actuality). We had dewetting on one batch of boards. Flatness of pads is great for fine pitch.

My questions are:

1. Does anyone have experinece with the shelf life on immersion white tin? If so, how long can the boards be stored without adversely affecting solderability. I've heard one year.

2. Does anyone have experience wave soldering white tin? If so, is the grainy appearance of the solder joints indicative of possible joint strength problems?

3. Has anyone experienced large variations in solderability from batch to batch of new white tin boards?

4. Does anyone using white tin require operators to wear gloves when handling these boards?

Would appreciate any comments.

Regards, Pat Pepper

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#7982

Re: Immersion White Tin Process for PCB Finish | 5 January, 2000

Hey Pat: My responses are:

1. Does anyone have experience with the shelf life on immersion white tin? If so, how long can the boards be stored without adversely affecting solderability. I've heard one year.

No, we don�t, because we turn boards faster than that. We�ve heard the same nice story. So, why do you want to store boards for a long time, anyhoo?

2. Does anyone have experience wave soldering white tin? If so, is the grainy appearance of the solder joints indicative of possible joint strength problems?

Yes. Alone the grainy appearance doesn�t mean much, although it (1) is not indicative of white tin and (2) is indicative of the approach you take to wave soldering. We wave solder shiny solder connections with white tin. Although, we could make the connections grainy, if we wanted to do that (which sometimes we do whether we�re supposed to or not). Of all the ways to make grainy connections, the easiest ways I know are: (the good) get the connection blistering hot and cool it slowly, (the bad) move the connection while it�s cooling, and (the ugly) wait before washing an aqueous flux. Check the archives for more background on grainy solder.

3. Has anyone experienced large variations in solderability from batch to batch of new white tin boards?

No, but with immersion tin being an electroless process, fabs do not control plating thickness well (same reason you�d be complaining about OSP thickness control, if you had selected that). So, if your fab�s a maroon and can�t control their process you�ll get poor solderability of thinly coated boards, same as when they�d mess-up the thickness of HASL boards. What kind of incoming solderability testing are you doing on boards?

4. Does anyone using white tin require operators to wear gloves when handling these boards?

No, we run aqueous flux. Either way, it�s good when operators try to keep their mits off solderable surfaces of boards, regardless. So, what�s the intent of the gloves? Are you running no-clean? Tell me, if someone scratches their nose while wearing gloves, ...

My2�

Dave F

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#7983

Re: Immersion White Tin Process for PCB Finish | 11 January, 2000

Hi Pat,

we had white tin boards laying around for about 10 month with no extra protection than the plastic bag that they were shipped in. Never thought of using them for there was a relayout planned to fix some serious bugs. But that�s life, relayout was cancelled and we had to build those things for prototyping and additional bugfix. The soldering results were bad in the first reflow cycle ( really bad wetting ). We send the remaining boards for stripping and reapplying a solderable surface to the boardhouse. In general I am not a "fan" of this finish cause with three soldercycles the wetting in the second cycle already wasn�t that good. Might look different with higher activated fluxes. We didn�t check for process variations for that�s not our favorite finish so I can�t say if that improves things.

In general: 1. Look at the storage conditions, (sealed drybags with dessicant) 2. Grainy appearance, follow Daves advise - 4. Daves point hits it

Not much but my experience

Wolfgang

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Pat Pepper

#7984

Re: Immersion White Tin Process for PCB Finish | 14 January, 2000

Hi Wolfgang,

Thanks for the response. We had a similar situation to yours with some white tin samples that sat around for about eight months. Samples soldered right after they arrived soldered better than the ones we soldered eight months down the road. But neither soldered very well, i.e., we saw "really bad wetting" as you did. We contacted the purchasing agent for additional samples from the same vendor, got them in, soldered them, and again had the grainy appearance and poor wetting. We notified the vendor and found out that their process had been running cool--white tin deposition is basically a time and temperture thing (electroless), and they had inadaquate process controls on their temperature--thus, the tin deposition was not thick enough (it's really an alloy, I'm told, not plating). The intermatallics grew and poor wetability resulted. As with anything, process control is crucial in the white tin process.

We like the flatness of the white tin pads for .4mm pitch QFP's. However, we seem to often be in the position of having boards around longer than we would like due to market conditions. We'll have to come to terms with this. Perhaps selective gold on an otherwise HASL board is the way to go? My understanding that the only drawback to selective gold is cost. Any thoughts?

Best, Pat

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Pat Pepper

#7985

Re: Immersion White Tin Process for PCB Finish | 14 January, 2000

Hi Dave,

Thank you for the response. In answer to your responses I'll use the original numbers of my questions.

1. We would like to turn around our boards fast, but sometimes due to the market conditions we wind up with inventory. After reading of Wolfgang's experience and knowing we have had similar problems, I'll have to get an action plan together on what to do with old boards so as not to loose the investment and still have a solderable board.

2. Thanks for the advice and info on grainy solder. I'll check the archives.

3. your point about the fab houses here is right on the money. See my note to Wolfgang for our experience with this.

4. We are running no clean--a very mild, low solids wave flux. I've heard that some folks require operators to wear golves on white tin jobs, others not. It seems to me that if you have a wetable surface, then gloves would not be necessary, although normal advoidance of touching solderable surfaces is in order.

Thanks again, Pat

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#7986

Re: Immersion White Tin Process for PCB Finish | 14 January, 2000

Yes selective gold might be a cost factor. How about the whole board in Ni/Au that�s what we do with no difference in costs to HASL or white tin. But maybe the pricing in Germany is different than in other places. Can�t say what�s the best for you but we live with that and our subcons too.

Regards Wolfgang

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#7987

Glove Me Tender, Glove Me True | 17 January, 2000

Continuing on the the discussion of using gloves in a no-clean soldering environment:

* Sure, some people use gloves when handling boards in a no-clean soldering environment * There may be some benefit to using gloves when no-clean soldering * You should understand the cleanliness requirements of your customer, the factor affecting this cleanliness in your plant, and the benefits of using gloves.

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