AVR-Chat

--- In AVR-Chat@yahoogroups.com, "Dave McLaughlin" <dave_mclaughlin@...> wrote:

> ... Graham ... have you considered some
> of the low cost prototype stencil offers
> that are now being made ... ?

Short answer, no.

My customers have a habit of delaying approval until the project can no longer be done by the required date.  As a result, on a paid project, I don't have the luxury of off-shore prototypes.  Usually, I use PCB Fab Express.  Either I pony up for a laser-cut stainless steel prototype (frameless) stencil from another source or I avoid parts that require a stencil.

I can handle 0.5mm pitch FPC connectors and ICs using the established process of depositing a thin continuous bead of solder paste across all the pads and removing bridges, if necessary, with de-soldering braid after reflow.

QFN devices I avoid entirely.  The first time I used them (with a stencil) they lifted up at a slight angle, perhaps due to outgassing of flux in the solderpaste on the die pad, and so the connections did not all solder.  Of course, once this has happened, it is not possible to correct it by hand.  Basically, I have lost my nerve.  I am waiting for an opportunity to piggy-back another attempt onto some other job that needs a stencil.

Regarding other stencil materials, for example, mylar, the laser raises a burr that in my opinion makes them unusable for fine pitch work.

Graham.