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From:
"Kane, Amol (349)" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Kane, Amol (349)
Date:
Wed, 22 Mar 2006 08:25:56 -0500
Content-Type:
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text/plain (182 lines)
Sure can,
Microcracking occurs to the mechanics of cooling of SAC305 alloy. To explain crudely, During cooling, Sn dendrites are formed starting from the barrel to the fillet surface. The remaining liquid alloy is then sucked into the spaces between these Sn dendrites. Concurrently, the substrate expands as it goes over the wave, and contracts when it cools. This creates stress in the joint when the solder is cooling. Due to this we see fissures in the fillet surface that is the last to cool (usually the bottom fillet)

I have an x-section picture of a microcrack...will upload it and post the link on the forum. IPC 610RevD says that the joint is acceptable if (a) the bottom of the crack is seen and (b) the crack does not touch the lead or the land

The problem is, you cannot see the bottom of the crack in the magnifications mentioned in IPC, and even if you can, the fissures do not form linearly into the fillet (the picture will make this statement clearer), and hence you may not be seeing the "true" bottom. IPC 610 Rev D tries to address this issue based on the info the industry as a whole has. Changes will occur to it in the future when more data is available, but for now we have the standard to go by.

I x-sectioned the joint and also x-rayed it to determine the extent of the crack in the fillet. Then based on a sampling plan, I am going to x-ray 'n' boards and if they are defect free, shall pass the lot. But do I want to keep doing this on lot of board I produce on the wave in the future?


Amol Kane
M.S (Industrial Eng.)
Process Engineer
Harvard Custom Manufacturing 
941 Route 38  Owego, NY 13827
Phone: (607) 687-7669 x349
[log in to unmask]

 -----Original Message-----
From:   TechNet [mailto:[log in to unmask]]  On Behalf Of arnaud grivon
Sent:   Wednesday, March 22, 2006 8:11 AM
To:     [log in to unmask]
Subject:        Re: [TN] mixing lead-free solders

Amol,

Could you expand on the kind of defects you experienced (cross section 
pictures) and clarify your "testing was a hassle" statements?

Phil Nutting a écrit :
> Thanks Amol.  I'll look at this more closely. 
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Kane, Amol (349)
> Sent: Wednesday, March 22, 2006 7:17 AM
> To: [log in to unmask]
> Subject: Re: [TN] mixing lead-free solders
>
> Hi Phil,
> Based on the info in the e-mail, you say you are using SAC305 in the
> wave.......did you do any testing that proved that SAC305 was superior
> than SN alloy in wave? Or is this an absolute requirement for the
> product/process?
>
> We used SAC 305 in wave and ended up with Hot Tears on greater than 90%
> of all TH joints........you may have the following issues
>
> 1. if you get hot tears, current IPC spec is not sufficient to pass/fail
> them (because it is impossible to see the bottom of any of the tears @
> magnifications mentioned in IPC 610)
> 2. I did some X-sectioning and X-ray analysis of our samples, and even
> though they turned out OK as per IPC, testing them was a hassle to say
> the least.
>
> So be careful before using SAC305 in wave.
>
> Good Luck
>
> Amol Kane
> M.S (Industrial Eng.)
> Process Engineer
> Harvard Custom Manufacturing 
> 941 Route 38  Owego, NY 13827
> Phone: (607) 687-7669 x349
> [log in to unmask]
>
>  -----Original Message-----
> From:   TechNet [mailto:[log in to unmask]]  On Behalf Of Whittaker, Dewey
> (AZ75)
> Sent:   Tuesday, March 21, 2006 4:36 PM
> To:     [log in to unmask]
> Subject:        Re: [TN] mixing lead-free solders
>
> I've seen several presentations that shows that to be an effective
> combination. All other things being equal, you should be fine.
> Dewey 
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Phil Nutting
> Sent: Tuesday, March 21, 2006 2:07 PM
> To: [log in to unmask]
> Subject: [TN] mixing lead-free solders
>
> Good afternoon,
>  
> As we move further into LF conversion I was curious what you, the gurus
> of TechNet, thought about the possibility of mixing LF solder alloys.
>  
> We are converting our manufacturing operation to SAC 305 for wave and
> hand solder.  We have a CM that is using Sn100C in their wave.  There is
> the potential to have to manually repair a board built with Sn100C by
> hand soldering with SAC 305.  Are there any reliability issues with
> this?  Assume the board will be properly cleaned after soldering.
>  
> I haven't looked into our SMT CM and their process, but it could be
> similar.
>  
> I'd hate to have to run three or four lines to keep each alloy separate.
> As it is I must manage Sn/Pb and SAC 305.
>  
> Thanks in advance.
>  
> Phil
>
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