PCB Assembly Services - Screaming Circuits: Caution from the parts drawer


Caution from the parts drawer

My recent post, "Law of Conservation of Parts" spoke to the value held in the old parts drawer. However, there are a few cautions that need to go along with that encouragement. Some parts do have a shelf life (batteries and electrolytic capacitors), but few have enough of a shelf life to be a concern in all but extreme cases.

I have some 7400 series logic chips in DIP form 7400 THthat I bought back in the early 1980s. Every now and then, I pull one out and put it into a proto board to test some circuit idea I've got. They still work thirty some years later. I haven't taken any special care in storage either. Some are stuck into anti-static foam or have oxidation on the pins. All are sitting in a mini-parts bin without any moisture protection. I guess they do get a little shielding from light, but basically, they're just hanging out. They've been, at various times, in the attic, in the basement, in the garage or in the house.

That may seem like good evidence refuting a shelf like for parts. And today's parts are even more robust Bent pins in stripto start with. Still though, two issues do come up: Moisture absorption and bending of pins.

When the component's destination is a robot pick and place machine and a 10 stage reflow oven, oxidation, moisture and bent pins can be the difference between the part being useable or not. Oxidation can prevent a good solder joint. Bent pins or missing BGA balls can prevent the part from fitting. Moisture absorbed over time can make the chip act like a popcorn kernel when in the reflow oven.

If you are planning on using old parts for a prototype, be sure to give them a good inspection before sending them off for assembly. And, if they're moisture sensitive parts or have been stored in high-humidity areas, ask your manufacturer to bake them before assembly. The same goes for raw PCBs too. Overly moist PCBs can delaminate during reflow. Some PCB finishes such as immersion sliver and OSP can tarnish or degrade over time too. Static sensitive parts that have not been stored in proper antistatic packaging may not be suitable for use - unless you have a way to reliably test.

Don't overlook your parts drawer if you can't find anyplace to buy a small quantity. Just look the parts over to make sure they are up to the task before dropping them in the shipping box. 

Duane Benson
Three bent pins
Three bent pins
I think there's a nursery rhyme in there somewhere

Once you have your parts, old and new (or are going to have us purchase them for you), head over to Screaming Circuits and get a quote.


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