It wasn’t too terribly long ago that just about any design could be built all through-hole. Okay, maybe it was a little longer ago than that. Once the big CPU chips stopped showing up in PGA (pin grid array), through-hole PC motherboard possibilities went out. Then when Bluetooth and ZigBee came around, most if not all of those chips came out in BGA, LGA or QFN forms — no through-hole. Anyway, it’s not too difficult to do pretty much any design in all SMT now, but what about all leadless?
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 its “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 need only 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 3.3V output and can accept 5 to 45V 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 through-hole or TSSOP and SOIC chips.
Oh wait. I work for a company that does that.
Duane Benson
Fight Uni!
BTCs are fine unless you want to solder and inspect them.
My company has spent the past 2 years developing stencil guidelines to make BTCs work but those guidelines are fluid and changing everyday.
Inspection is a nightmare. Xray is the only possibility but many of the xray machines currently in factories have problems with BTCs. Looking at a component with no solder fillet and no solder height has proven to be one of the most difficult components ever to xray.
I hope that the new IPC standard addresses some of these issues.
The other problem this increased packaging density adds; is thermal density. While efficient at getting the heat from the die in these packages, the size reduction means engineers can add extra circuitry to boards or reduce board sizes, increasing the thermal density of designs and adding to the PCB designers woes!!!