How Do You Prototype with QFP-Only Components?

Question

I'm starting to design a circuit that includes a QFP-38 component. I'm accustomed to using DIP components and prototyping on a breadboard, but in this case I won't be able to acquire a DIP version of the component.

Is it normal practice for electrical engineers to have a PCB fabricated without prototyping? Or do you always acquire breakout boards for small components at first?

(In my case, I would just go ahead and order a QFP-38 to DIP adapter, but it will cost me $20 including shipping. It seems like I may as well just have a PCB made and hope for the best.)

Answer 1

I make my own PCBs at home, it's very easy and doesn't cost much. I can make a board in 30 minutes.

Another option is to use a breakout board for the QFP, like these Schmartboards.

Answer 2

I just have a PCB fabbed. Its incredibly cheap to do so nowadays unless you need exotic materials or construction parameters.

Making your own PCB is reasonable for hobby work but when contracting for $150-200+ an hour as most design houses do, having a 4 board 4 day turn done is just cheaper than screwing around with adapters or etching your own copper.

Answer 3

I've used dead-bug style prototyping for such parts.

The basic idea is that you start out with a copper-clad board which is all copper, then glue down the parts with their pads sticking up, using cyano.

This way you can easily solder to the pads and pins of small parts using wirewrap wire or other fine wire and you will have a good solid ground plane anywhere you want it, which is good for noise immunity.

Islands of non-ground connections are made by cutting out some small (like 3x3 mm) bits of PCB and gluing those down with the copper facing up, I've even mounted some SMD parts this way.

Answer 4

If I want to tiker with one isolated component that has not too much pins (let's say less than 40), I use a breakout board. When more is involved it becomes impractical, because you have to hook so many wires from the breakouts to the breadboard that the mess turns it highly probable that you'll make a mistake. What I do sometimes is make a "partial" board, in which I have the high-density components interconnected and hook just their ends (usually one or two SPI ports) to the breadboard. I do the boards myself on the first design round, and have it fabbed when I arrive to the final design - or sometimes when the prototype board is too complex and I'm unable to make it myself.

I know two good ways to make PCB at home. One is with toner transfer paper and the other is with photosensitive PCBs.

With the first you print your PCB to that special paper using a laser printer, and then press this paper against the board using a heat source (can be an old iron). After that you use water to wash out the paper and the toner is attached to the copper, ready to etch.

With the photosensitive board you print the circuit to a transparent sheet, place it on the board and expose it to UV light; then you apply a developer and again it's ready to etch.

The first method is cheaper (assuming you already have a laser printer) but I found the limit for vias to be about 0.5 mm. With the second you can have 0.3 mm.

Some links:

http://www.pulsarprofx.com

http://www.injectorall.com

Answer 5

Sometimes, such a QFP device will have either a demo board that you can buy with headers broken out, or have a family equivalent device in a smaller/bigger package (which either is available in DIP or have an easy to prototype demo board).

This will let you get started in a hurry and validate your design, and let you get on the software while you are still working on designing/building the "real" PCB.