Re: Anisotropic conducting films and pastes

Sep 06, 2003 2 Replies
G


After doing a google search of the newsgroups, this has hardly been


> mentioned so I will give a little background from what I have learned.
>
> An anisotropic conductor is one that will conduct in say the z
> direction, but not in the x and y directions. It is has been most
> commonly used in the manufacture of lcd displays to attach the wires
> to the lines. It is now being considered for use in mounting BGA's and
> flip chip components.
>
> Basically it works by having small metallic particles embedded in a
> film/tape or a paste. When the component is pressed against the pad,
> the particles get trapped between the two pads and complete the
> connection. The whole component is heated to cure the tape or paste so
> the component stays in place.
>
> The nice part is that the process is relatively low temperature
> (~200C) and removeable with solvent or low heat.. Easier than
> umounting and reballing a BGA that did not solder right.
>
> Has anyone experimented with these materials for hobbyist use for
> mounting BGA chips. Anyone know where a roll of this anisotropic
> conducting film could be bought. It seems like this would be a method
> to allow the hobbyst to experiment with BGA and other high pin density
> components that are so common now.

You've got my attention, I'll be waiting for an answer too.

Vote
Twitter Facebook LinkedIn
R

PCBexpress will sell you a metal stencil to allow you to "screen-print" solder paste onto your PC board, place the SMD, and then reflow in a

Vote
Twitter Facebook LinkedIn
G

I know about PCBExpress's stencils, but I don't believe they say anything about it being possible to do BGAs with that method. For regular SMDs I don't have any difficulty just using a hot-air reflow system ($20 heat gun) and fine-tipped soldering iron.

This anisotropic stuff would basically be a thin sheet of material that you would cut to size and place between a BGA and the pad grid. With a little bit of heat, the material softens a bit and allows an adhesive bond and the BGA balls embed themselves into the material. Then the anisotropic properties ensure that conductivity from each ball only goes straight down, where the corresponding pad is located.

Several things that must be considered:

1) Conductivity. Most similar material I've seen is a plastic or rubber with conductive material mixed in. There's some resistance there which might be a problem, especially with power connections.

2) Thermal conductivity (related to above). If the BGA heats up at all, there is no metal path for heat to wick away from the device. May require more attention to heatsinking.

3) Thermal stability. If the film bonds at a low temperature, will it become soft if the BGA produces heat, or the board is in a hot environment? Obviously 200C is well above operating limits, but I'd wonder if it would get soft enough to slowly shift.

4) Positioning accuracy. A major benefit of BGA is that you can be as much as half a ball-width away from target, and the ball surface tension effect will realign the device. With this film, you'll need to be able to accurately position the device without the benefit of self-positioning. This can end up being a challenge, considering the size of many BGA devices.

Vote
Twitter Facebook LinkedIn

Join the Discussion

Have something to add? Share your thoughts — no account required.

Didn't find your answer?

Ask the community — no account required