Influence of copper surface protective layer on le

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The impact of copper protective layer on lead-free solder joint structure of printed circuit board (Part I)

with the approaching of the deadline of the EU decree on all electronic and electrical products entering the market must meet the lead-free requirements in July 2006, the lead-free process of electronic products has attracted more and more attention from manufacturers. Although many studies have shown that lead-free assembly is feasible and has some favorable characteristics, there are still many problems to be solved in the electronic assembly industry before it is completely lead-free, one of which is how to better ensure the effective compatibility between printed circuit boards and lead-free alloys. From the perspective of electronic properties, Sn - Ag - Cu is considered to be a promising alloy system to replace eutectic Sn - Pb. This alloy has good wettability on the copper surface, can form good solder joints, and its thermal mechanical properties are also better than eutectic Sn - Pb alloy. In order to maintain good wetting characteristics with Sn - Ag - Cu in the assembly process, it is necessary to carry out surface treatment on the copper surface of the printed circuit board before assembly. The treatment method is to add protective layers, such as organic protective layer (OSP), silver immersion layer (im2mersion AG, I - Ag), gold immersion layer (immersion Au, I - AU), electroless plating ni/au layer, etc., of which organic protective layer and silver immersion layer are mostly used, However, there is no report on the effect of these two protective layers on the structure of lead-free solder joints. Therefore, combined with the process of lead-free surface mount technology in actual production, the influence of copper surface protective layer of printed circuit board on the structure of lead-free Sn - Ag - Cu solder joint was studied

1 experimental process

the alloy composition is Sn - 3 0 %Ag - 0. 5%cu (mass percentage), lead-free solder paste with rosin flux, put 1 The 2 mm thick printed circuit board and ball grid array BGA components are welded according to the surface mount technology process, that is, the vacuum packed printed circuit board is opened, printed through the solder paste template, and then passed through the reflow soldering furnace. The copper surface of printed circuit board adopts two different protective layers, namely organic protective layer and silver impregnated layer. During reflow soldering, the temperature of the insulation zone is 2, which is in an unstable state of 18 ℃, and the peak temperature is 235 ℃. After reflow soldering, a series of lead-free BGA/cu welding samples were made. The BGA is separated from the printed circuit board by tensile test, and then the changes of the fracture mode and fracture of the solder joint are observed under the scanning electron microscope to determine the influence of the copper surface protective layer on the fracture mode of the solder joint. The tensile rate is 0 5 mm/min。 In order to determine the influence of the copper surface protection and perfect superalloy technology system layer on the solder joint structure, the BGA/cu solder sample was cut at the same position of the printed circuit board, and after polishing, the (HNO3) 5% + (HCl) 2% + (H2O2) 0 5% mixed acid solution is used for corrosion treatment, and then 25% nitric acid alcohol solution is used for micro corrosion. Different welding samples after corrosion treatment were observed and analyzed by jeoljsm6360 scanning electron microscope (SEM) with Inca EDs

2 test results and analysis

after the tensile test of lead-free solder joints protected by organic protective layer, a large number of solder joints broke near the copper surface, as shown in Figure 1 (a), showing that the strength of lead-free solder joints is lower than the bonding strength of copper surface and printed circuit board; The observation of the fracture surface of the broken solder joint shows that there are many pores in the solder joint, as shown in Figure 1 (b). These pores improve the energy utilization efficiency of the plastic granulator process and prevent environmental pollution, which are complementary in different sizes. Langsheng is still investing in the development of Changzhou factory, and there are flux residues in the pores. After the lead-free solder joints protected by silver dipping layer passed the tensile test, most of the copper surfaces were pulled out, as shown in Figure 1 (c), showing that the strength of the solder joints was higher than the bonding strength between the copper surface and the printed circuit board; The observation of the broken solder joint shows that as shown in Figure 1 (d), the fracture surface is relatively flat and there are relatively few pores

the main reason for the pores in the organic protective layer solder joint is that the organic protective layer decomposes when passing through the reflow furnace to produce gas, which forms pores near the interface because it is too late to discharge. When the solder joint is subjected to external force, stress concentration occurs at the pores and microcracks are formed. Then the microcracks expand and grow, and finally lead to the fracture of the solder joint, so the strength of the solder joint is reduced

Figure 1 fracture scanning electron microscope analysis of lead-free solder joint after tensile test

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