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When Pb-free meets FR4

Posted: 17 Oct 2008     Print Version  Bookmark and Share

Keywords:Pb-free  FR4  delamination  organic solder protectant  conductive anodic filaments 

Figure 1. High reflow temperatures can cause vertical stresses that can rupture copper barrel platings.

Damaged copper barrel platings
The same expansion in the z dimension that causes delaminations can also stretch copper barrel platings vertically. It's not uncommon for the barrel plating to break under these conditions. The break can occur on the weakest point along the length of the barrel.

Conductive anodic filaments
This condition can occur when temperatures are high and in a location where two plated through-holes are relatively close to each other. "In any printed circuit board—it's woven fibreglass cloth that's been dipped in epoxy—there can develop fine cracks between the epoxy and the fibreglass," Dr. Lasky says.

"If you have two through-plated holes that are 10 [thousandths] or 20 thousandths of an inch away from each other, and if they have a different electrical potential, over time you can actually get a plating of copper along the microcracks in the glass. And you can actually get a filament that grows from one electrode to the other, causing a short."

Figure 2. The possibility of conductive anodic filament (CAF) growth between adjacent through-holes increases as reflow temperature rises.

Conductive anodic filaments become much more likely when the reflow temperature is above 240°C, he notes. He adds, however, that the problem may be less serious and less frequent than is ordinarily thought. "I'm not really sure how many fails have been attributed to this," he says. "There could be a little bit of an over-reaction."

Sagging
Somewhat related to warpage and delaminations is the tendency of some FR4 boards to sag at higher temperatures. Dr. Lasky explains, "We're talking about temperatures that are basically 20 degrees hotter than leaded solder, and for plastics these temperatures are amazingly hot." A board that has no problem with sagging at 220°C may have a real problem at 240°C, he observes, and at 260°C the board may sag so much that it could fall off the rail. To prevent sagging, some assemblers place something like a piece of piano wire in the centre of the rails.

-Tom Adams
Consultant
Sonoscan Inc.


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