With true ENIG (as defined in IPC-4552), any copper dissolution is rare. With ENIG, the dissolution (IMF) is of the nickel plating into the solder. Very seldom does the nickel completely dissolve and allow any copper IMCs to form. The gold surface is completely dissolved almost immediately upon initial contact with the molten solder.
Upon removing a BGA and soldering a new one on, the IMF is formed directly to the existing nickel/solder IMF left behind on the board pads after removal and site prep. The additional IMF thickness increase is very small, typically less than 1 micron, because the dissolution of tin or tin/lead into the nickel has already reached equilibrium during the original soldering and site prep process. Once equilibrium is reached, very little additional IMF growth occurs. Typically, the copper is never involved, although very minor amounts will show up in the IMF also.
Odin

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Victor Hernandez
Sent: Wednesday, September 16, 2015 9:09 AM
To: [log in to unmask]
Subject: Re: [TN] BGA Reworked Intermetallic

On ENIG surface I don't see much of an increase in the IMC formation thickness.  However, on Cu it is a different story.   I have measured IMC formation greater tham 10 microns.   Not sure of the below statement about E-NI parameter.   Please explain!!!

Victor,

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Vladimir Igoshev
Sent: Wednesday, September 16, 2015 8:12 AM
To: [log in to unmask]
Subject: Re: [TN] BGA Reworked Intermetallic

The "magic" Number should stay the same 1-3 micron but you'd also have to keep an eye on what happened to the layer of E-Ni underneath.

Regards,

Vladimir

SENTEC

Sent from my BlackBerry 10 smartphone on the Rogers network.
  Original Message
From: Datacom - Juliano Ribeiro
Sent: Wednesday, September 16, 2015 09:09
To: [log in to unmask]
Reply To: TechNet E-Mail Forum
Subject: [TN] BGA Reworked Intermetallic

Hi to all,



When we reworked the BGA, removed the component of the board and replacement another BGA, what's the intermetallic thickness ideal after the rework?



p.s: Our pcb is ENIG finished and the solder is Tin Lead.



_____________________________

Juliano Bettim Ribeiro

DATACOM

ENGENHARIA DE PROCESSOS
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+55 (51) 8446-2135

+55 (51) 3933-3000

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