# DDR4 BER Degradation Due to Crack in FBGA Package Solder Ball
## 1 Introduction  
DDR synchronous dynamic RAM (SDRAM) has evolved over the years in response to ever increasing demand for speed and lower power consumption. DDR4 has data rates from 1600 Mbps to 3200 Mbps and has a rated voltage of 1.2 V [1]. The increased data rate results in smaller bit interval, which produces stringent timing requirements. The lower operating voltage of DDR4 reduces the margin for voltage noise in the system. The timing and voltage effects due to defects in the system which were not significant before are becoming more important with the increase in data speeds. DDR4 uses fine pitch ball grid array (FBGA) packages due to their higher input output terminals density, low package dimensions and less parasitic effects [2, 3, 4]. These packages have solder balls under the package, which are soldered directly to the printed circuit board. The inspection of solder balls becomes difficult once a device is soldered to the printed circuit board. The reliability of the interconnection between the device package and the printed circuit board remains the weakest link in the reliability of the system. The interconnection is susceptible to faults due to mechanical and thermal stresses in the operating environment. FBGA package solder balls are susceptible to defects, such as void, crack, and fractures. Voids in solder balls occur during the reflow process and they reduce the mechanical reliability of the solder balls [5]. Cracks in solder balls occur due to thermal coefficient mismatch between the device package and the printed circuit board. The mismatch causes stress on the solder ball and results in cracks in the solder ball [6, 7, 8, 9]. These cracks are cumulative in nature and may develop into fractures over the period of time. The crack does not cause an open in the interconnection between the device package and the printed circuit board. However, it changes the electrical characteristics of the interconnection. The crack introduces a parallel capacitance in the electrical model of the interconnection [10, 11]. The value of the capacitance depends on the dimensionsof the crack and the solder ball. The parallel capacitance attenuates higher frequencies and increases the rise time of the signal. The data eye opening becomes smaller due to degradation of the signal. As the data rate increases there is more degradation of the signal. This decrease in the width of the data eye opening degrades the bit error rate of the data channel and, thus, reduces the reliability of the system. To see the effect of crack in solder ball on the signal integrity of the DDR4 data channel, the data channel is loaded with a capacitive element [12]. The value of the capacitive element corresponds to the dimensions of the crack. The data eye is measured for the channel with the capacitive loading and is compared with a channel having no defect. The data eye is used to create bathtub plots for the crack defect data channel and the no defect data channel. The comparison of the bathtub plots show the BER degradation due to crack in solder ball [13,14]. Section 2 details the crack defect in FBGA solder ball. The electrical model of a non-defective solder ball and a solder ball with crack defect are presented. The crack in solder ball adds parallel capacitance to the electrical model of the solder ball. The capacitance values due to crack are calculated for cracks of various cross sectional areas. Section 3 details simulation setup and simulation measurements for a DDR4 data channel having cracked solder ball. The data eye width and voltage margin losses are calculated. The simulations show data eye degradation due to crack in solder ball. The simulations are done for data rates of 2.133 Gbps, 2.666 Gbps and 3.2 Gbps. It is shown that as the data rate increases the data eye width and voltage margin losses increase. Section 4 details the measurement setup and test fixtures for inspecting the crack effect in a running system. DDR4 RDIMM module is loaded with a capacitor to mimic the crack in solder ball.