Low Alpha

Low and ultralow alpha-emitting semiconductor assembly materials are now essential for flip-chip packaging and are also becoming increasingly critical for power semiconductor assembly, as smaller active device sizes and thinner wafers increase the devices' sensitivity to ionizing radiation.



An alpha particle is an ionized particle consisting of 2 neutrons and 2 protons emitted spontaneously from the nucleus of specific isotopes of certain high atomic weight  elements. Of particular interest and concern to semiconductor fabricators and packaging houses are the elements uranium (U) and thorium (Th), isotopes of which decay to give stable isotopes of lead (Pb), but which, by decaying, give rise to alpha particles.

For over a year now, we have been supplying low alpha (LA) and ultralow alpha (ULA) emitting solder pastes to Asian customers. During this time, we have faced numerous challenges: most importantly:

-        The absence of standards in the area of measurement and test methodology

-        Metrology at the ultralow alpha level is hampered by a signal to noise ratio of about 1:1, where the “noise” is simple background radiation, present at around the 0.002cph/cm2 level.

Even the definitions of the different levels are not well defined, but we are operating under the working rule that:

-        LA =< 0.020cph/cm2

-        ULA =< 0.0020cph/cm2

I had the opportunity to discuss our work at the latest Second Annual IEEE-SCV Soft Error Rate (SER) Workshop in late October this year - please follow the link to learn more -  and I’d like to thank Peng Su of Cisco for giving me the opportunity to speak at this meeting. Also look for the forthcoming (at time of writing) review paper “Challenges in Supply of Ultralow Alpha-Emitting Solder Materials” by me and Olivier Lauzeral of iROC Technologies in Chip Scale Review magazine for November/December 2010.

In the coming months, Indium Corporation will be providing LA and ULA solder alloy, preform, solder powder, solder paste and flux materials.

Cheers! Andy

Solderspheres or solder spheres, or even solder balls: whatever you call them, Indium Corporation has been making them for years and has rightly acquired the reputation for doing whatever it takes to meet our customers' unique needs.

Unique Alloys:

Hard to find alloys (like multipart alloys; low-melting alloys and even gold/tin (80Au/20Sn)) are our bread and butter. As "Indium Corporation" it should be no surprise that we lead the world in our ability to supply low-melting indium-alloy solder spheres, as well as other forms of these alloys, such as engineered solders or solder pastes.

Unique Quantities:

We don't want you to buy more than you absolutely need. If you just want 100 spheres, we can easily do that: if you want more - we can do that, too. But remember that, because each customer's need is unique, our prices may be higher than our competitors, especially for more standard alloys. Some customers also have unique inventory-control needs, so we work with many customers to ship on-demand by retaining a buffer stock of spheres here at Indium.

Unique Sizes:

Our current dimensional capabilities as of this writing are from 80microns to 0.062inches, or even bigger. Generally, the bigger the sphere - the less spherical it is (within the limits of surface tension and viscosity), and we can't control the laws of physics, so instances where a very large amount of solder is needed, a preform may have better dimensional control. Also, notice that we won't ask that you order in a specific unit of diameter measurement, like the mil or the micron or the millimeter: we're a global company - just tell us what you need.

Unique Packaging:

Often needed for more delicate alloy spheres, we can offer specialty overpacking that eliminates oxides from the atmosphere around the solder spheres, essentially stopping oxidation in its tracks. It's the same technique we use to package our soft solder die-attach (SSDA) wire: a technique that showed that the very reactive wire was still "as new" 3 years later. We also offer spheres in tape & reel packaging (see image) for 24mil, 35mil and 62mil diameter spheres.

Unique Tolerances:

Just as a case in point, a MEMS customer of ours had a need for a low-melting indium-alloy solder sphere with a tolerance of +/-5microns (+/-0.005mm) for a sphere with a 350micron diameter. That demands a tolerance of just over 1% - pretty demanding, but we did it.

Our standard tolerance is +/-1mil (1 thousandth of an inch, or 25.4microns), but as you can see, we have the capability to go to much tighter tolerances using three proprietary manufacturing techniques.

Other Needs:

We are also seeing people asking for doped-alloy spheres; low-alpha emission solder spheres and other things that we could never have dreamed of...

So please just let us know what you need. We'd be happy to help out, and if we can not do what you ask - we'll let you know why.

Cheers!  Andy

 

• Are you looking for information on semiconductor packaging materials?  Send your request to jhisert@indium.com.  It’s all fair game – released, experimental, or competitor materials.  Flux characteristics, paste properties, application methods…

   

  Inquire about any of the following topics:

  • Pin transfer
  • Package-on-Package (PoP)
  • Solder spheres
  • Glass transition temperatures (Tg)
  • Flip chip assembly
  • BGA rework
  • Cross-sectioning electronic components
  • Paste for component dipping
  • Solder alloys
  • Liquid fluxes
  • Wafer bumping
  • Low alpha solder
  • Spin coating
  • Redistribution layers (rdl)
  • Halogen-free
  • Flux viscosity
  • Solder paste viscosity
  • Whatever else you are interested in

As discussed yesterday, alpha emission can disrupt sensitive electronic components. One source for alpha emission is the solder that is used to form interconnections between the substrate and chip. There are ways to decrease the alpha emission, as stated in a white paper by Indium’s very own Dr. Lee.

Alpha particles are emitted from certain element isotopes, and can potentially change the binary output of a semiconductor from a 0 to a 1 or from a 1 to a 0. This is a cause for concern in high reliability devices.

Alpha emission is measured in counts/(square centimeter x hour). This measurement refers to the amount of particles that are collected by a sensor over a given amount of time. This measurement falls into various categories such as low count class 1 (LC1) 0.2-0.5 count/centimeter x centimeter/hour.

Tomorrow I will describe how solder suppliers like Indium Corporation can help lower alpha particle emission within an electronic component.