Corrosion in Die and Package 

  

Corrosion is the degradation of metals as a result of electrochemical activity.  The process of corrosion requires 4 components for it to occur:  1) an anode; 2) a cathode; 3) an electrolyte; and 4) electrical connection between the anode and the cathode.  Thus, the key to corrosion prevention is the elimination of at least one of these components. The presence of an anode and a cathode implies that there is a potential difference between them, i.e., the anode has a greater tendency to lose electrons while a cathode has a greater tendency to gain them.  The presence of this potential difference is the primary driver of corrosion.

          

The anode is the metal or site with a higher potential to oxidize (lose electrons).  Thus, a metal undergoing corrosion is said to be 'anodic' if it is where the oxidation reaction takes place:  M → Mx+ + xe- .  The anode is often one of the following:

1)  the more active metal;

2)  a stressed region such as a crack, a scratch, a grain boundary, or a deformed structure;

3)  an area that is starved of oxygen; and

4)  an area with variations in its composition.

        

The cathode is the metal or site with a higher potential for reduction (gaining of electrons), or lower tendency to oxidize.  It is often one of the following:

1) a noble metal;

2) an unstressed region;

3) an area with high oxygen concentration; and

4) a non-metallic component.

           

The electrolyte is the medium through which ions may move, the most common of which is water.

 

Die Corrosion   

                    

Die corrosion refers to the corrosion of the metal areas on the surface of the die. Aluminum (Al) metal areas are the most prevalent on a typical die circuit, so Al corrosion is quite commonly encountered. Other thin-film layers on the die such as sichrome resistors can also corrode. Gross cases of chemical corrosion can lead to either electrically open or electrically shorted metal lines, with the latter being due to corrosion byproducts that can bridge two metal lines together.  Corroded metal lines appear dark under an optical microscope (as shown in the picture on the right).

       

Chemical corrosion of Al is triggered by the presence of moisture and contaminants on the die surface.  Corrosion of Al can occur whether it is acting as an anode or as a cathode. Al bond pads, being unglassivated, are more vulnerable to corrosion. However, corrosion can also occur in subsurface Al lines that are accessible to moisture by imperfections in its protective glassivation or inter-metal dielectric layers. 

                      

Corrosion is often a result of many wafer fab or packaging contamination problems.  Improper rinsing or excessive use of corrosive contaminants such as P, S, and Cl during wafer fab can make the die highly susceptible to die corrosion.  Packaging and passivation defects that allow excessive ingress of moisture and contaminants into the die can also lead to die corrosion. The use of plastic molding compounds with corrosive ingredients and the use of die attach material that exhibits resin bleeding of corrosive contaminants may likewise trigger die corrosion.   

         

Fig 1. SEM photo of corroded aluminum metal lines

Fig 2. Photo of a corroded bond pad that exhibited ball lifting; this corrosion was caused by Cl contamination

             

Lead/Leadframe Corrosion

         

Lead corrosion, as the name implies, refers to the corrosion of the lead itself. Lead corrosion is often due to inadequate lead finish, the presence of contaminants on the leads, and exposure of the leads to excessive moisture.  It can be accelerated by higher temperatures and the presence of electrical bias on the leads.

         

Leadframe corrosion refers to the corrosion of any part of the leadframe. Although this mechanism becomes more critical if it occurs on the silver-plated areas (die pad where the die is set and the bonding fingers) of the leadframe, corrosion on any part of the leadframe must be rejected because the contaminants in the corroded area can easily spread in the presence of moisture. 

           

The most frequently encountered contaminants in fresh leadframes are chlorine, phosphorus, sulfur, and potassium.  Newly-delivered leadframes from suppliers must undergo strict incoming quality screening for contaminants/foreign materials to minimize the risk of internal corrosion in semiconductor products.

          

Fig 3. SEM photos of corroded areas on contaminated die pads of various leadframes

       

Wire Corrosion

      

Corrosion of the bond wires within a package can also occur, gross cases of which can lead to wire breaking or even total disintegration of the wire. This mechanism is more commonly encountered in aluminum wires that have been contaminated by chlorine, although rare cases involving gold wires have also be observed. 

       

In the case of gold wires, delaminated areas around the wire can act as conduit of Cl-contaminated moisture that can expose the entire length of the wire to Cl and make it vulnerable to massive corrosion.  Other contaminants that accelerate gold wire corrosion include bromide, iodide, and cyanide ions. 

                

Fig 4. SEM photo of a corroded aluminum wire and wedge bond

   

See Also:  Oxidation/Reduction Potential ValuesPackage FailuresDie Failures;

Failure AnalysisBasic FA Flows Reliability Models

 

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