J Fail. Anal. and Preven.(2008)8: 524-532 525 investigation [11 a series of modern analytical instruments and methods such as stereomicroscope, optical microscop (OM), and digital microscope(3D)were used to observe the failure. The scanning electron microscope(SEM), energy dispersive spectroscopy(EDS), and focus ion beam(FIB were adopted to reveal the cracking morphologies and chemical compositions of the vias. Characterization tech niques such as Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analyses(TGA), and scanning acoustic microscopy (SAM) were also used to inspect the bare board. The cracking mechanism and the causes of the ⊙⊙ failure were definitely revealed. Preventive countermea sures and suggestions are given in this paper Brief Introduction of the Failed PcBs (a) Figure i shows the circuit configuration of both sides omponents on two types of PCBs, respectively. dimension of92×37×1. I mm for type I 81 38 x 0.98 mm for type 2. Open-circuit faults have been detected mainly around BGA solder joints and solder bars of several components and chips by automatic optical inspection(AOD). The faults induce failure of the whole board. During macroscopic inspection, circuit configura- tions of components on type 1 PCB are not distorted, but obvious warpage has been found in type 2 PCB. Modern analytic instruments and characterization methods have ⊙.⊙· been adopted in order to find the primary causes. Characterization and microstructural observation (b) The results of FT-IR(Fig. 2)show that the two types of PCb Fig. 1 Circuit configuration with components on PCBs (a)fror have an FR-4 flame-resistant substrate and that the raw back face of Type I PCB. (b) Front and back face of Type 2 materials are all bromized epoxy resin. TGA curves in Fig. 3 indicate that the materials reach their thermal decomposi- reflow soldering or service. Moreover, the mismatch of tion temperature(Ta) at about 323C. The Ta of bromized coefficient of thermal expansion( CTE) between the plating epoxy resin is generally above 300C, and the soldering layer and copper foil will enhance the thermal stress, peak temperature in surface mounting technology(SMT)of finally resulting in the fracture from one side of the via PCB is below 270C. Therefore, the raw materials can The location of the blind via in Fig. 5 indicates that the via endure the instantaneous high temperature and the failure and the BGa solder pad are too close to each other, just should have no relationship to the soldering processes. like the case shown in Fig. 4. An obvious microvoid in the Microscopic study was carried out at low magnification copper foil, in which there appeared to be some debris, and using stereomicroscope and at higher magnification using an apparent crack in the bottom of the via, are displayed in PCBs From Fig 4(c)and(d), distinct cracking can be seen observed by SEM, and chemical compositions along the in the center of the via. Consequently, a strong thermal and the microstructure near cracking is presented in Fig. 8 stress will occur around part of the via, leading to an SAM with a sensor of 100 mhz was used to detect asymmetric thermal deformation under conditions of delamination of the PCB before and after the thermal 2 Springerinvestigation [11], a series of modern analytical instruments and methods such as stereomicroscope, optical microscope (OM), and digital microscope (3D) were used to observe the failure. The scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), and focus ion beam (FIB) were adopted to reveal the cracking morphologies and chemical compositions of the vias. Characterization tech￾niques such as Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analyses (TGA), and scanning acoustic microscopy (SAM) were also used to inspect the bare board. The cracking mechanism and the causes of the failure were definitely revealed. Preventive countermea￾sures and suggestions are given in this paper. Brief Introduction of the Failed PCBs Figure 1 shows the circuit configuration of both sides with components on two types of PCBs, respectively, with dimension of 92 9 37 9 1.1 mm for type 1 and 81 9 38 9 0.98 mm for type 2. Open-circuit faults have been detected mainly around BGA solder joints and solder bars of several components and chips by automatic optical inspection (AOI). The faults induce failure of the whole board. During macroscopic inspection, circuit configura￾tions of components on type 1 PCB are not distorted, but obvious warpage has been found in type 2 PCB. Modern analytic instruments and characterization methods have been adopted in order to find the primary causes. Characterization and Microstructural Observation The results of FT-IR (Fig. 2) show that the two types of PCB have an FR-4 flame-resistant substrate and that the raw materials are all bromized epoxy resin. TGA curves in Fig. 3 indicate that the materials reach their thermal decomposi￾tion temperature (Td) at about 323 C. The Td of bromized epoxy resin is generally above 300 C, and the soldering peak temperature in surface mounting technology (SMT) of PCB is below 270 C. Therefore, the raw materials can endure the instantaneous high temperature and the failure should have no relationship to the soldering processes. Microscopic study was carried out at low magnification using stereomicroscope and at higher magnification using OM and 3D. Figures 4 and 5, respectively, present the morphologies of the failure blind vias on both types of PCBs. From Fig. 4(c) and (d), distinct cracking can be seen between the copper plating layer and the pad of the blind via. As illustrated in Fig. 4(b), the pad is not located right in the center of the via. Consequently, a strong thermal stress will occur around part of the via, leading to an asymmetric thermal deformation under conditions of reflow soldering or service. Moreover, the mismatch of coefficient of thermal expansion (CTE) between the plating layer and copper foil will enhance the thermal stress, finally resulting in the fracture from one side of the via. The location of the blind via in Fig. 5 indicates that the via and the BGA solder pad are too close to each other, just like the case shown in Fig. 4. An obvious microvoid in the copper foil, in which there appeared to be some debris, and an apparent crack in the bottom of the via, are displayed in Fig. 5(c) and (d). Cracking morphologies of the blind vias were further observed by SEM, and chemical compositions along the fracture line were analyzed by EDS in Figs. 6 and 7. A FIB was used to etch part of the surface of the deposited copper, and the microstructure near cracking is presented in Fig. 8. SAM with a sensor of 100 MHz was used to detect delamination of the PCB before and after the thermal Fig. 1 Circuit configuration with components on PCBs. (a) Front and back face of Type 1 PCB. (b) Front and back face of Type 2 PCB J Fail. Anal. and Preven. (2008) 8:524–532 525 123