Flip Chips and Flashlights
by George A. Riley, Contributing Editor
With the industry’s attention riveted on the next-generation of TSV—enhanced stacked—everything 3D marvels, we sometimes forget how microelectronics are changing everyday products in our world.
When a New England ice storm left my neighborhood without electricity for a cold week last December, flashlights went from household conveniences to survival necessities. I was pleased to find that research I was aware of several years ago for applying microelectronic assembly techniques to LED packaging has led to a new world of electronic flashlights with capabilities that your grandfather’s two D-Cell electric torch can’t match.
At the 2005 SMTA Emerging Products session which I chaired, Lumileds, the high-intensity LED pioneer, presented the conference’s award-winning paper on flip chip packaging of high-power LEDs. The thermal conductivity advantage of gold stud bump assembly over solder allows higher LED operating temperatures, and therefore higher light output.
Even the lower power of small LED pocket flashlights requires careful attention to heat removal. The usual cooling problem is not just how do we remove the heat, but where do we put it? In a ring array, LEDs are distributed around the periphery of the flashlight cylinder, moving the heat sources away from the center and apart from each other, while placing them at the heatsink— the aluminum case.
One might expect a beam-forming problem in this scattered arrangement, but LEDs have several beam-forming advantages over the old-fashioned bulb. The plastic lens integral with each LED can be shaped to provide a radiation pattern ranging from Lambertian (center-weighted), to a broader beam, to side-illuminating.
Beyond that, the palm-sized personal flashlights we survived with turned the potential drawback of a six-LED ring into an advantage, producing a radiation pattern perfect for our needs, but not obtainable with ordinary bulb flashlights.
While a strongly focused spotlight beam is splendid for locating distant objects, it can be awkward for stumbling around in a pitch-black cellar. Think of the difficulty of using the ultimate focused beam, a laser pointer, to navigate that cluttered space. The ideal light for that purpose would combine a focused straight-ahead main beam with a wide-angle, dimmer but diffuse light for general awareness —as our LED flashlights did.
The light reflector positions a flat, patterned aluminum disk behind the ring of LEDs, with the highly-reflective vertical wall of the aluminum cylinder surrounding the ring. This simultaneously provides the best of both worlds: a beam about 6 feet in diameter, and a diffuse, uniform light emitted over about160 degrees horizontally and vertically.
Finally, the overall design meets our smaller-cheaper semiconductor obsession. The one-inch diameter aluminum case with a tail- cap push switch and a removable AAA battery pack is 3.5 inches long. The flashlight fits comfortably in the palm, and conveniently in a pocket, weighting about half as much as a D-size battery.
The filament-free LED adds robustness; LED efficiency promises longer battery life. Most surprising, a package with 6 of these flashlights and 18 batteries costs less than 20 dollars, even for panicked crowds of in-the-dark buyers. Three cheers for electronic flashlights, and the microelectronic packaging that makes them possible!