Low Alpha Lead Sources Are Emerging
John Baliga, Associate Editor -- Semiconductor International, 1/1/1999
One concern for upcoming technology generations is soft upset generation caused by lead in flip-chip solder connections. This is due to alpha particle emission from the lead. Alpha particles carry a positive charge and are highly energetic. These 5 MeV alpha particles are capable of producing over 106 electron-hole pairs within 25µm of travel through silicon.
The lead used in flip-chip connections will have to be refined to reduce alpha emission to a very low level, much below the ambient cosmic level. This is because the lead will be in constant, close proximity to the chip. Also, if the lead is to be delivered in a powder form, its alpha emission must be extremely low.
The alpha particle emission from lead actually comes from the decay of one of its daughter elements, polonium, from 210Po to 206Pb (see Figure). The removal of polonium from lead is a simple chemical process. The challenge in providing low-alpha lead is removing 210Pb from the lead sample. 210Po decays with a half-life of ~138 days. It is produced by the decay of 210Pb, which has a half-life over 22 years. If left in the sample, 210Pb would act as a long-term source of 210Po.
 |
| Fig. 1. The radioactive decay chain from 210Pb to 206Pb ends with 210Po decaying into 206Pb and an alpha particle. The half-lives for each transition are shown. (Source: Pure Technologies) |
The entire process of producing the low-alpha lead must be exhaustively controlled from mining through final delivery. When making PbSn solder, the tin should be certified as low-alpha as well. Additional sources of radioactivity can be introduced very easily. In fact, one instance of alpha-induced soft upset was traced to an air jet, contaminated with a small amount of alpha-producing material, used by a nitric acid vendor to clean bottles1.
| Company News | |
|
Intarsia (Fremont, Calif.) and Philips Components (San Jose, Calif.) announced plans to collaborate and jointly develop standards for integrated passive devices in chip scale packages. Siemens Semiconductors (Munich, Germany) and ESEC (Cham, Switzerland) announced a joint test, assembly and packaging integrated architecture (TAPIA) project to develop and implement manufacturing automation hardware and software. The first pilot installation is scheduled for the end of 1999 at Siemens Components Pte. Ltd. (Singapore). White Electronic Designs (Phoenix) introduced an EDO DRAM that integrates two die in an 89-ball BGA package. W.L. Gore's (Eau Claire, Wis.) Via on Chip Pitch package has been qualified by NEC Corp. for use in flip-chip BGA packaging of ASICs. Coors Ceramics Co. will not relocate its Chattanooga, Tenn.,
facility. | |
Pure Technologies (Atlanta) uses laser isotope separation technology to remove 210Pb. This method was developed by Lawrence Livermore National Laboratories (LLNL) to separate 238U from uranium. The sample is heated to a vapor, which rises between two electrostatic plates. A dye laser is used to ionize the atoms of the desired isotope, which are then collected on the plates. Dye lasers are used to provide highly controlled laser frequencies, which are selective enough to ionize atoms of one isotope and not another.
Johnson Matthey Electronics (JME, San Diego) is also making plans to supply low-alpha lead. The company has not specified its refinement method. JME is supplying it as a Type 6 powder with alpha emission values <0.02 cph/cm2.
For any sample of lead to be certified as 'low-alpha,' it must be monitored
for a number of months. Suppliers use monitoring times ranging from three to six
months to make sure the concentration of 210Po is not increasing. In
addition to the two companies mentioned above, others have been offering
low-alpha materials. Standards for measurement methods and emission levels will
need to be established. 
References
- J.F. Ziegler et. al., 'IBM Experiments in soft fails in computer electronics (1978-1994),' IBM J. Resl Develop., Jan. 1996, p. 3.
