PCB Assembly Services - Screaming Circuits: All Leadless


All Leadless

It wasn't too terribly long ago that just about any design could still be built all thru-hole. Okay, maybe it was a little longer ago than that. Once the big CPU chips stopped showing up in PGA (pin grid array), thru-hole PC motherboard possibilities went out. Then when blue-tooth and Zig-bee came around, most if not all of those chips came out in BGA, LGA or QFN forms - no thru-hole. Anyway, it's not too difficult to do pretty much any design in all SMT now, but what about all leadless?

DFN-8 Okay, we don't really consider passives to be leadless, but they kind of are. So, we have all of our passives in a leadless like form. Now all we have to worry about are the chips.

I'll start with a Microchip PIC18F4550 in a QFN44 package. It's got built-in USB, so I don't have to worry about a separate USB chip. I'll load up a bootloader and it will all be happy. Wireless will have to wait for version 2.0. This is going to control a two side-by-side wheel platform scooter type thing, so I'll need a gyro and accelerometer. Digi-Key just sent out their "techzone" mini-catalog/magazine featuring just some of these type parts. I'll take the Analog Devices ADXL345 three-axis accelerometer in LGA form-factor.

I only need to worry about pitch and yaw, so a dual axis gyro should be fine. I'll try out the ST Micro LPY550AL in a 5x5mm QFN package. For voltage regulation in the prototype, I'll use a Linear Technology LTC3642 in a 3x3mm DFN package. It has a 3.3 volt output and can accept 5 to 45 volts in. That gives me the flexibility of powering off of a dedicated battery pack or off the scooter main battery.

All LGA or QFN/DFN. The only problem is soldering up the prototypes and next half-dozen or so units, for all of my friends, after that. I'm not going to stick those things in a toaster, and I certainly can't hand solder them like I could with the old thru-hole or TSSOP and SOIC chips. Oh. Wait. I work for a company that does that.

Duane Benson
Fight Uni!


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Do you have a 6x8 Eagle package for this chip?

Hi David;

Your very question is one we've been trying to answer for a while. The way most other companies reduce costs is either by building higher volumes (which won't help you) or by making quality the result of luck rather than process.

We have a lot of systems in place to ensure consistent high quality with very complex jobs. Those systems and processes do make small jobs more expensive. That's perfect for commercial projects but I do understand that it does frequently take us out of the running for self-funded hobby projects.

Unfortunately, last I checked, I couldn't get in the door for <$3-400 for even one QFN. Is there any way to make that sort of tiny run feasible for a hobbyist and for you?

You would be amazed what a skilled tech (or hands-on design engineer) can do with a hotplate, a bottle of flux, a scope and a good touchup iron.

QFN - no big deal.
LGA - same thing, sometimes ground pads are tricky.
BGA - this is where I hand the board over to the professionals!

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