Issue link: https://iconnect007.uberflip.com/i/1533904
48 SMT007 MAGAZINE I APRIL 2025 e static refresh t ref or retention time is the amount of time a DRAM cell or device can reliably hold data; and several studies 4-6 have reported degradation in the DRAM reten- tion time upon exposure to radiation such as X-rays. Measurement of other timing parame- ters, such as t ac (access time) and t rp (precharge time), had been made in prior X-ray exper- iments; however, no significant shis were observed. As such, only t ref appears to be the most sensitive parameter. Since leakage is a strong component of refresh characteristics, it is believed that the increase in reverse junction leakage is the most probable cause for t ref degra- dation aer subjecting DRAM compo- nents to X-ray radiation 1 . e DRAM damage due to irradi- ation poses a significant challenge to the inspections of BGAs with on-pack- age DRAM to produce an image qual- ity that is needed for providing the nec- essary information regarding the phys- ical defects present, e.g., voids in sol- der joints aer SMT assembly. As BGA pitch reduces, it is becoming increas- ingly necessary to go to higher magni- fications, which reduce the distance between the package and the X-ray source and hence leads to higher dosage. In addition to devel- oping methods to inspect the packages, it is also crucial to characterize the functional performance of DRAM upon X-ray exposure. is paper provides an in-depth analy- sis of the factors influencing X-ray inspec- Table 1: DOE table for radiation exposure Filter Material Filter thickness (mm) Inspec on me (sec) Tube Voltage (kV) X-ray power (W) # of Runs Comments No Filter 0 55 120 14.5 2 Filter material & thickness Aluminum 1.6 55 120 14.5 2 Zinc 1.6 55 120 14.5 2 Copper 1.6 55 120 14.5 2 Aluminum 4.8 55 120 14.5 2 Aluminum 4.8 135 120 14.5 2 Inspec on Time Aluminum 4.8 255 120 14.5 2 Aluminum 4.8 315 120 14.5 2 Aluminum 4.8 75 160 20.0 1 Tube Voltage & Power Aluminum 4.8 75 140 14.5 1 Aluminum 4.8 75 160 14.5 1 Aluminum 4.8 75 140 17.5 1 No Filter 0 75 160 20.0 1 No Filter 0 75 160 14.5 1 Figure 1: Functional test set-up for DRAM performance. Figure 1: Functional test set-up for DRAM performance.