How the Apple iPhone 4 drives high, low tech

SAN JOSE, Calif. – Apple's iPhone 4 pushed the envelope both in its use of 3-D chip stacks and by-hand system assembly, according to a presentation by UBM TechInsights.

The iPhone 4 was among the first mainstream smartphones to use as much silicon area as printed-circuit board area, thanks to aggressive use of 3-D chip stacking. The handset also had an unusually high number of small assemblies and screws, presumably requiring intensive manual labor assembly similar to that used on high-end wrist watches, said David Carey, a teardown expert at UBM TechInsights.

The net result was a system in which "the electronics virtually disappear," Carey said in a talk at the Linley Mobile Tech Conference here. The chips are among the few components that can be miniaturized in mobile systems "opening up as much internal volume as possible for the battery and display" which cannot be shrunk, said Carey.

The iPhone 4 used a small L-shaped circuit board tucked around its battery. Creating such a small, dense design pushed more complexity to the circuit board itself. The handset used a ten-layer board with tiny connections between layers made by lasers and photo-imaging rather than traditional drilled vias.

The board was "basically a macro version of structures and processes of we see in [semiconductor] chip layers," Carey said. "It looks like an IC cross section and is much more expensive than a traditional circuit board," he said.

The techniques had a small, but significant extra cost that Carey estimated at about a penny per layer per square centimeter of board space. The net result was a board that cost about $5, a $3-4 premium per unit over some handsets.

"It's table stakes for that form factor," said Carey. "Apple doesn’t have to compete on price, so their handsets can cost more to make--they have that luxury," he said.

3-D chip stacks are widely used in smartphones, but few handsets have the 1:1 ratio of silicon of the iPhone 4. Many use multilayer pcbs with micro-vias, but few use ten-layer boards, Carey said.

"We first saw a ten-layer handset board in a Nokia pen-sized phone five or six years ago," he said. "Every smartphone we look at is at least partly down this path" but few are at the extreme of the iPhone 4, he added.

Cross section of the iPhone 4 board    Source: UBM TechInsights