Liquid IC/SiP encapsulant, base frames? - Page 1
Forum section: electronics / general

author: Spirit532
author_rank: Frequent Contributor
timestamp: December 24, 2018, 10:08:21 pm
content: I'm currently working on a very interesting project that may be able to(hopefully) revolutionize a product niche and bring the price down to a true consumer level, cutting product BOM costs by at least 40%The device I'm designing is intended to be used as a part on regular PCBs, and I would like to manufacture small quantities of directly solderable surface-mount modules. However, the particular requirements of this device bring a few questions to the table:Are there any existing products that could serve as a system-in-package base frame? The size of this device would fit perfectly in a QFP100 footprint(or even smaller), but a lot thicker(10-15mm), however switching to a wire-bonded process(and using real leadframes) is impossible due to a number of factors(see below). There's always the obvious solution of making a multi-layer PCB with either castellated holes or BGA pads on the bottom, but I would like it to be easier to solder, for prototype purposes if anything. It only needs to have 5-8 electrical connections, none high speed, and it should be fairly easy to assemble on standard PnP equipment. Originally I envisioned it as a QFN-like part, like those fully integrated DC-DC converters, but it seems that the upfront investment for that kind of package is too much.Do there exist liquid IC/SiP encapsulating compounds that can be bought in small amounts(<1l)? I need a liquid, room temperature(or slightly above, but not a lot) curing compound, preferably with a low viscosity. The device has a temperature sensitive liquid(optical silicone) and a fragile component that may crack or shift out of alignment during an injection molding or thermoset molding process, so I can't use regular granular compounds and pressure/heat. It needs to be light-tight at thicknesses above 1.5-2.5mm. At the moment I'm thinking of using a regular PCB potting compound, but I'm curious as to whether there exist any special compounds for this specific purpose.This is pretty much exactly what I'm hoping to achieve:However, it's easy to spot even by the shape that this has been injection molded or at the very least under extreme pressure, the package clearly has a large draft angle to not get stuck in the mold.This may be an option, from what it appears it's just a PCB with pins that's been potted in a regular plastic enclosure.At this point the final product design is nothing more than a vague idea in my head, but functional prototypes are being evaluated and are ready for miniaturization(currently 30x30x10mm, the goal is 10x10x10mm). I'd be grateful if anyone who has had experience with this could chime in and help me set more realistic expectations and design goals.

author: SeanB
author_rank: Super Contributor
timestamp: December 24, 2018, 10:52:19 pm
content: Large draft angles are more for low pressure moulding, to make it easy to get the part out than anything else. For small quantities go to the auto parts shop or your local boat dealer and get a small pot of polyurethane resin and catalyst, or go to the local hobby stores and get some 2 part casting resin, which is optically clear and very liquid till cured. Signage supplies places will have acrylic monomer and the catalyst for it as well, used to make signage and to join sign sheeting, and they supply this in 100ml and 500 ml cans by me, with a separate bottle of catalyst for it that turns it from something like thin syrup to a solid block in a few hours. For the light tight use there are acrylic colourants that you can add to the resin, must be available in black, though that probably is going to be slightly conductive. Otherwise for the polyurethane resin there are also colourants that can make black, and you just mix them in to the base before catalysing it. Cheap will be to use a 2 part epoxy like Pratley quickset steel, which is opaque and somewhat liquid till it cures.Leads you can just use the castellations on the PCB, and a small PVC potting box made from some tape and PVC sheet for the prototype units, you can have a better silicone mould made later on for low volume runs. Otherwise just some wire ends will work on SM pads under the potting. Volume wise there are companies who will, for a cost, stamp out a leadframe for you.

author: Spirit532
author_rank: Frequent Contributor
timestamp: December 24, 2018, 11:13:24 pm
content: I know how to do it in a "DIY hobby" style, but the methods mentioned are barely suitable for a run of 30, let alone 100 pieces. That's what I'm after. I'm looking for commercial potting solutions. And no, clear epoxy won't do, this needs to be completely opaque without being conductive(so again, no DIY carbon-based additives).