Hi Leland,
I'd suggest you section both soldered leads (old and new) and see if there is any difference composition wise
Vladimir
SENTEC Testing Laboratory Inc.
11 Canadian Road, Unit 7.
Scarborough, ON M1R 5G1
Tel: (416) 899-1882
Fax: (905) 882-8812
www.sentec.ca
-----Original Message-----
From: Leland Woodall <[log in to unmask]>
Sender: TechNet <[log in to unmask]>
Date: Tue, 27 Mar 2012 21:16:12
To: <[log in to unmask]>
Reply-To: TechNet E-Mail Forum <[log in to unmask]>,
Leland Woodall
<[log in to unmask]>
Subject: Re: [TN] Rework Concern and Question
Richard,
The leads of both components were supposed to be coated with pure tin. The lead finish was not supposed to change. Our solder paste used for reflow has always been SAC 305.
We've tried to remove the new component at three different soldering stations. The irons are all calibrated and checked each morning.
We've fluxed the joints and use tweezer attachments as the removal tool, and have scrapped 10 boards in the process.
Something has changed, and the only change we've been able to identify is at the SEM.
Leland
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Tuesday, March 27, 2012 5:01 PM
To: [log in to unmask]
Subject: Re: [TN] Rework Concern and Question
I think, but am not sure, that you are attempting to remove a new component whose leads are FINISHED (not soldered) with SAC305 solder, and that is why you measure Sn98 on the parts straight out of the tape (I am going to disregard the SEM analysis of 2% silicon for a moment).
Secondly, the new parts were probably soldered to the PWB with SAC305 solder as well, and that is why when you attempt to unsolder them or bring them into liquidus, you are having more difficulty than you did with the old parts.
Not only were the old parts finished with Sn63 solder, but they were also soldered in place with Sn63 solder.
The new parts are meant to be RoHS compliant, and apparently they are, with a Sn level of 98%, and no Pb in sight on the SEM diagram. If that were the only change, you should notice no difference in the ability to remove the part if they were, in fact, soldered to the PWB with Sn63; the plating change alone would not make any noticeable difference.
Soldering them to the PWB with lead-free SAC305 alloy would make a huge difference in your ability to unsolder and remove the part, no matter what finish was used to plate the part.
Silicon will not break down and become part of an alloy under 1000 deg. C. The 2% SEM pickup is probably silicon chips or dust from the component body.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Wenger, George M.
Sent: Tuesday, March 27, 2012 3:25 PM
To: [log in to unmask]
Subject: Re: [TN] Rework Concern and Question
Dave Hillman,
With your metallurgical background you might be a better person than I to make a comment. I'm not sure I've even had or seen an occasion where something dissolved in the bulk solder and increased the melting/solidification temperature. Yes I've seen the leading edge of solder as it flows and wets a surface dissolve metallization and freeze because the local meting temperature changed but I've never seen that happen to the bulk of a solder joint
Regards,
George
George M. Wenger
Senior Principal Reliability / FMA Engineer Andrew Corporation - Wireless Network Solutions
40 Technology Drive, Warren, NJ 07059
(908) 546-4531 Office (732) 309-8964 Mobile
E-mail: [log in to unmask]
-----Original Message-----
From: Leland Woodall [mailto:[log in to unmask]]
Sent: Tuesday, March 27, 2012 4:14 PM
To: TechNet E-Mail Forum; Wenger, George M.
Subject: RE: [TN] Rework Concern and Question
George,
It's been a long day, and maybe I'm not making myself too clear.
We're not being able to achieve a liquidus state with a 700 degree F soldering iron. We're not trying to melt the lead, just the solder that's holding it to the PCB. We're not accomplishing that with the new component, so I'm thinking something has mixed with the solder joint and has raised the resultant reflow temperature.
Leland
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Wenger, George M.
Sent: Tuesday, March 27, 2012 4:10 PM
To: [log in to unmask]
Subject: Re: [TN] Rework Concern and Question
Leland,
Am I missing something? I don't think it really matter that the lead base material or lead surface finish is. What matters is the solder used to attach the lead to the board. When you desolder a lead you don't melt the lead you melt the solder.
Regards,
George
George M. Wenger
Senior Principal Reliability / FMA Engineer Andrew Corporation - Wireless Network Solutions
40 Technology Drive, Warren, NJ 07059
(908) 546-4531 Office (732) 309-8964 Mobile
E-mail: [log in to unmask]
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of David D. Hillman
Sent: Tuesday, March 27, 2012 4:05 PM
To: [log in to unmask]
Subject: Re: [TN] Rework Concern and Question
Hi Leland - I recommend you find out the lead base metal composition. If you shot an SEM-EDS of the lead toe, you could be getting the Si from the base metal composition and not part of the solder alloy composition.
Dave Hillman
Rockwell Collins
[log in to unmask]
Leland Woodall <[log in to unmask]> Sent by: TechNet <[log in to unmask]>
03/27/2012 02:52 PM
Please respond to
TechNet E-Mail Forum <[log in to unmask]>; Please respond to Leland Woodall <[log in to unmask]>
To
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Subject
Re: [TN] Rework Concern and Question
Vladimir,
We analyzed raw components straight out of the tape and reel packaging.
The site selection was on the bottom side of the leads at the toe.
Leland
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of vladimir Igoshev
Sent: Tuesday, March 27, 2012 3:12 PM
To: [log in to unmask]
Subject: Re: [TN] Rework Concern and Question
Sorry Ben, it doesn't.
Leland,
What you are saying sounds strange. What does it mean: "We've analyzed the lead material beneath the SEM and it returns 98% tin and 2% silicone"? Did you analyzed leads or solder? Where the analysis was taken from? Did you analyzed leads with pads ripped off?
Regards,
Vladimir
SENTEC Testing Laboratory Inc.
11 Canadian Road, Unit 7.
Scarborough, ON M1R 5G1
Tel: (416) 899-1882
Fax: (905) 882-8812
www.sentec.ca
-----Original Message-----
From: "Gumpert, Ben" <[log in to unmask]>
Sender: TechNet <[log in to unmask]>
Date: Tue, 27 Mar 2012 18:58:21
To: <[log in to unmask]>
Reply-To: TechNet E-Mail Forum <[log in to unmask]>,
"Gumpert, Ben"
<[log in to unmask]>
Subject: Re: [TN] Rework Concern and Question
Leland,
I won't claim to be an expert, but this website seems to imply something along those lines.
http://resource.npl.co.uk/mtdata/phdiagrams/sisn.htm
Ben
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Leland Woodall
Sent: Tuesday, March 27, 2012 2:18 PM
To: [log in to unmask]
Subject: EXTERNAL: [TN] Rework Concern and Question
Folks,
We've ran across a strange incident and I'd like a little advice from the group.
We recently underwent a component vendor change, and part of the first group of boards were misbuilt due to incorrect polarity (the part marking was misinterpreted by the vision operator).
Anyway, an attempt to remove the part by our Repair group resulted in lifted pads on 10 of the first 12 boards. We've analyzed the lead material beneath the SEM and it returns 98% tin and 2% silicone. The old style component comes off quite easily (within 3 seconds), and an analysis of its leads shows to be 100% tin. It's a 6 pin diode with very little mass.
What's going on here? Does a 2% silicone mix raise the melting point of solder by 200 degrees C?
Please help me understand.
Thanks,
Leland
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