J Fail. Anal. and Preven. (2010)10: 399-407 Activation Step The activation bath in this case was electrode's reduction reaction reaches certain over poten- smaller than regular bath generally used and the activation tial value, the hydrogen evolution reaction occur. The time was about 10-12 s compared with only 5 s in the differential between hydrogen evolution potential and regular bath. Also, the current density per unit area was hydrogen equilibrium potential is called"hydrogen evo- igher than normal value. Large density current and longer lution over potential"and n is used to denote it. The value exposure time means that more hydrogen was generated of n is determined by Tafel,s equation on the to current passing through the stainless steel belt and hydro- gen is given by Faradays Law [16] In which, both a and b are constants and the value of blog j could be ignored in most metals, i.e., the value of n equals Q=(1-n) a. In acidic solution, the value a is 0. 70 for steel, compared with the value of 0.87 for copper and 1. 20 for tin. The In this equation, Q is the generated quantity of hydrogen, lower a (i.e, the hydrogen evolution over potential k is the current efficiency, I is the current, t is the acti the more easily hydrogen evolution reaction occurs. The vation time, and 96500 is the Faraday constant. In this step, reaction formulas are given as follows [18, 19] hydrogen evolution is primary the side reaction, $0 Cathodic reaction: H++e-H, H+H-H2 (Eq 5) (1-k) almost equals the efficiency of hydrogen genera tion. In the same section of activation step, the current and density is proportional to I. This equation demonstrates that a lot of the hydrogen generated was due to the increased time of activation and larger density current. Additionally, Anodic reaction: Sn -Sn++2e (Eq7) hydrogen evolution in activation step could extend to deoxidation and electroplating steps Reaction (5)takes place on the surface of stainless belt, and(6)takes place on the surface of chip lead frame Deoxidation Step Electrolysis pickling is used in deoxi dation step and will generate additional hydrogen Sulfuric Hydrogen Embrittlement acid is used in deoxidation step from IC analysis(Fig. 3). Commonly, there are four main reactions which may tak ke Hydrogen atoms usually enter material by means of place on the stainless steel belt surfaces when exposed to adsorption and dissociation of hydrogen molecules [20].It sulfuric acid is known that fatigue and hydrogen-induced fracture gen- Fe2O3+ 3H2SO4-Fe2(SO4)3+3H2O (Eq 1) hydrogen can enter stainless steel surfaces rather than be Fe304+4H2SO4-FeSO4 +Fe2(SO4)+4H20(Eq 2) eliminated by the surface oxide layer. The hydrogen mol FeO+H2SO4→FeSO4+H2O ecules reach the metallic surface and are transported to the crack tip and absorbed. There are two factors involved in Fe+H2SO4→FeSO4+2H (Eq 4) this absorption: one is physical adsorption of the hydrogen molecules and the other is chemical absorption of nascent As the solubility of Fe2(SO4)3 is smaller than FeSO4, hydrogen(protons)and movement of the nascent hydrogen reaction (1)and (2) are the slow ones comparing with nto the steel (absorption). These two processes can be reaction(3)and (4). The surface of stainless steel belt is explained as two key steps, shown in Fig. 11 [22] covered by oxide layer which is rich in Cr], and protects Hydrogen molecules must dissociate into two hydroger the stainless steel surfaces. Therefore the aforementioned atoms in order to enter the stainless steel [23]. Hydrogen reactions would not take place until the oxide layer was atoms penetrate into the belt and are trapped at defects such broken. Once the surface passivation layer is broken, the sulfuric acid enters cracks in the layer and reacts with as carbides, sulfides, and grain boundaries. These trapped the exposed Fe, Cr, Ni alloy and enlarges and deepens the ecules at microvoids inside the steel. After forming H2 cracks. The generated hydrogen can enter stainless steel belt through cracks [17. molecules, the hydrogen pressure within the microvoids will increase and may cause blistering and/or bond break- Electropoiung Step As we know, hydrogen evolution ing, i.e., result in hydrogen embrittlement on the material reactions occur when the rate of the hydrogen reduction In fact, hydrogen pressure relates to the reaction is higher than that of hydrogen oxidation. These two rates are equal in equilibrium reaction. In other words, CH=135 PH2 nIy SpringActivation Step The activation bath in this case was smaller than regular bath generally used and the activation time was about 10–12 s compared with only 5 s in the regular bath. Also, the current density per unit area was higher than normal value. Large density current and longer exposure time means that more hydrogen was generated in this step. The quantity of hydrogen depends on the total current passing through the stainless steel belt and hydro￾gen is given by Faraday’s Law [16]: Q ¼ ð1 gkÞ I t 96500 In this equation, Q is the generated quantity of hydrogen, gk is the current efficiency, I is the current, t is the acti￾vation time, and 96500 is the Faraday constant. In this step, hydrogen evolution is primary the side reaction, so (1 gk) almost equals the efficiency of hydrogen genera￾tion. In the same section of activation step, the current density is proportional to I. This equation demonstrates that a lot of the hydrogen generated was due to the increased time of activation and larger density current. Additionally, hydrogen evolution in activation step could extend to deoxidation and electroplating steps. Deoxidation Step Electrolysis pickling is used in deoxi￾dation step and will generate additional hydrogen. Sulfuric acid is used in deoxidation step from IC analysis (Fig. 3). Commonly, there are four main reactions which may take place on the stainless steel belt surfaces when exposed to sulfuric acid. Fe2O3 þ 3H2SO4 ! Fe2ð Þ SO4 3 þ 3H2O ðEq 1Þ Fe3O4 þ 4H2SO4 ! FeSO4 þ Fe2ð Þ SO4 3 þ4H2O ðEq 2Þ FeO þ H2SO4 ! FeSO4 þ H2O ðEq 3Þ Fe þ H2SO4 ! FeSO4 þ 2H ðEq 4Þ As the solubility of Fe2(SO4)3 is smaller than FeSO4, reaction (1) and (2) are the slow ones comparing with reaction (3) and (4). The surface of stainless steel belt is covered by oxide layer which is rich in Cr2O3 and protects the stainless steel surfaces. Therefore, the aforementioned reactions would not take place until the oxide layer was broken. Once the surface passivation layer is broken, the sulfuric acid enters cracks in the layer and reacts with the exposed Fe, Cr, Ni alloy and enlarges and deepens the cracks. The generated hydrogen can enter stainless steel belt through cracks [17]. Electroplating Step As we know, hydrogen evolution reactions occur when the rate of the hydrogen reduction reaction is higher than that of hydrogen oxidation. These two rates are equal in equilibrium reaction. In other words, only when the equilibrium potential of hydrogen electrode’s reduction reaction reaches certain over poten￾tial value, the hydrogen evolution reaction occur. The differential between hydrogen evolution potential and hydrogen equilibrium potential is called ‘‘hydrogen evo￾lution over potential’’ and g is used to denote it. The value of g is determined by Tafel’s equation: g ¼ a þ b log j In which, both a and b are constants and the value of blog j could be ignored in most metals, i.e., the value of g equals a. In acidic solution, the value a is 0.70 for steel, compared with the value of 0.87 for copper and 1.20 for tin. The lower a (i.e., the hydrogen evolution over potential) is, the more easily hydrogen evolution reaction occurs. The reaction formulas are given as follows [18, 19]: Cathodic reaction: Hþ þ e ! H; H þ H ! H2 ðEq 5Þ and Sn2þ þ 2e ! Sn ðEq 6Þ Anodic reaction: Sn ! Sn2þ þ 2e ðEq 7Þ Reaction (5) takes place on the surface of stainless steel belt, and (6) takes place on the surface of chip lead frame. Hydrogen Embrittlement Hydrogen atoms usually enter material by means of adsorption and dissociation of hydrogen molecules [20]. It is known that fatigue and hydrogen-induced fracture gen￾erally initiates at the material surface [21] and that hydrogen can enter stainless steel surfaces rather than be eliminated by the surface oxide layer. The hydrogen mol￾ecules reach the metallic surface and are transported to the crack tip and absorbed. There are two factors involved in this absorption: one is physical adsorption of the hydrogen molecules and the other is chemical absorption of nascent hydrogen (protons) and movement of the nascent hydrogen into the steel (absorption). These two processes can be explained as two key steps, shown in Fig. 11 [22]. Hydrogen molecules must dissociate into two hydrogen atoms in order to enter the stainless steel [23]. Hydrogen atoms penetrate into the belt and are trapped at defects such as carbides, sulfides, and grain boundaries. These trapped hydrogen protons will accumulate and may form H2 mol￾ecules at microvoids inside the steel. After forming H2 molecules, the hydrogen pressure within the microvoids will increase and may cause blistering and/or bond break￾ing, i.e., result in hydrogen embrittlement on the material. In fact, hydrogen pressure relates to the hydrogen concentration: CH ¼ 135 ffiffiffiffiffiffiffi pH2 p 6500 RT J Fail. Anal. and Preven. (2010) 10:399–407 405 123