TechNetters,

 

I had an off TN email with Victor about IMC and I’ve since realized I probably should have shared it with all of the TechNetters.  The below email is that exchange.

 

George

 

From: David Hillman [mailto:[log in to unmask]] 
Sent: Wednesday, May 17, 2017 9:29 AM
To: George Wenger <[log in to unmask]>
Cc: [log in to unmask]
Subject: Re: [TN] IMC Formation Thickness in OSP

 

Hi guys! Victor - what George wrote should be in a text book! Perfect treatise on how to view IMC! 

 

George - great words!

 

Dave

 

On Wed, May 17, 2017 at 8:23 AM, George Wenger <[log in to unmask] <mailto:[log in to unmask]> > wrote:

Hi Victor,

 

As usual Dave Hillman and I think a lot  alike when it comes to soldering.  I’ve included him in this email.  It looks like he was doing the same thing I was doing (i.e., waiting to see what the TechNetter responses would be).  

 

I agree with his comments about IMC.  In my simple way of thinking I don’t think IMCs are a problem.  Actually an IMC is a necessity because it is the “glue” layer that holds solder unto copper.  The thickness of an IMC isn’t a problem as long as there is solder above it and copper below it.  The real problem with an IMC is that when an IMC becomes oxidized it isn’t solderable with the types of fluxes that are used in the electronics industries.  IMC became a problem (talked about a lot) when the electronics industry wanted a really flat surface to solder to because they began using lots of fine pitch components.  When tin (Sn) comes in contact with copper either during a plating process (i.e., immersion tin) or when molten solder comes in contact with copper (HASL or hand soldering) an IMC forms.  Typically the thickness of the IMC is 0.25 to 0.75 microns thick (10 to 30 micro-inches).  Because of solid state diffusion the ICM grows with time and the growth accelerates with temperature.  If the IMC consumes all of the tin above it (i.e., it grows to the surface), the IMC is exposed to the environment (oxygen) and it oxidizes.  And as  said oxidized IMC isn’t solderable with types of flux used in the electronics industry.  This issue was a real problem with Immersion tin plating.  When immersion tin plated PCBs were taken out of storage there were lots of cases when the immersion tin wasn’t solderable because the IMC had drown to the surface an oxidized.  The old specification for immersion tin didn’t call out an immersion tin thickness and the plating thickness was often in the 10 to 30 micro-inch range.  I actually saw one of the early specs in my previous companies (Andrew/CommScope) that said the immersion tin plating should be “minimum”(i.e., thick enough so that the copper surface had a silver color).  I’ve seen more than one case were immersion tin PCBs were taken out of stock after only a couple of weeks of storage and when they were put through a typical soldering process there were many poor and “no solder” joints.  

 

In my Lucent Technology days we also had problems with HASL boards when we asked the board vendor to make the HASL “flat”.  To make it flat they blow really hard and actually blow off all of the molten solder and left behind the IMC which oxidized an wasn’t solderable when we went to solder it.

 

If you look at the IPC spec for immersion tin (IPC-4554) you will see a change that occurred several years ago where they now call for a minimum immersion tin thickness of 1 micron (40 micro-inches).  This it to insure that the immersion tin will be solderable after one year of storage which means the IMC would not have grown to the surface and oxidized.

 

So IMC thickness in itself isn’t a problem.  You just want to make sure that there is solder above the IMC and copper below the IMC

 

George

 

From: [log in to unmask] <mailto:[log in to unmask]>  [mailto:[log in to unmask] <mailto:[log in to unmask]> ] 
Sent: Wednesday, May 17, 2017 6:57 AM
To: [log in to unmask] <mailto:[log in to unmask]> 
Cc: [log in to unmask] <mailto:[log in to unmask]> 
Subject: RE: [TN] IMC Formation Thickness in OSP

 

Dell - Internal Use - Confidential 

George,

 

    Thank you very much for sharing your experience.   What are the Pros & Cons of too thick of an IMC formation, copper dissolution.

 

Victor,

 

-----Original Message-----
From: George Wenger [mailto:[log in to unmask]] 
Sent: Tuesday, May 16, 2017 4:59 PM
To: 'TechNet E-Mail Forum' <[log in to unmask] <mailto:[log in to unmask]> >; Hernandez, Victor G <[log in to unmask] <mailto:[log in to unmask]> >
Subject: RE: [TN] IMC Formation Thickness in OSP

 

Victor,

 

I haven't seen and TechNetter responses but I'll give you my two cents.  The IMC thickness numbers you heard sound much too high.  I think the IMC thickness should be half as thick as those numbers.

 

 

Regards,

George Wenger

 

 

 

-----Original Message-----

From: TechNet [ <mailto:[log in to unmask]> mailto:[log in to unmask]] On Behalf Of  <mailto:[log in to unmask]> [log in to unmask]

Sent: Tuesday, May 16, 2017 8:35 AM

To:  <mailto:[log in to unmask]> [log in to unmask]

Subject: [TN] IMC Formation Thickness in OSP

 

Fellow TechNetters:

 

   What is the acceptable guideline for IMC Formation thickness on OSP after

2x rework..   IPC-4555 is still pending.  I've heard of 3-5 microns on first

pass and 5-10 microns on 2x RW.   All comments/suggestions welcomed.

 

Victor,