EAGLE CAD Custom Parts Gotchas

Here are some notes and reminders to myself, things to double and triple-check when creating a custom part.

These are things that I’ve messed up.

Package Creation

EAGLE Layers

Top (or Bottom) means exposed copper on the board, use the “Land Pattern” information in the datasheet to create these pads:

Land pattern = the exposed copper

tStop (bStop) means no colored solder mask applied in this area, so the copper is cleanly exposed, usually a bit larger than the copper pads:

qfn24-solder-mask-is-tstop

tCream (bCream) shows the size of the stencil openings, usually a bit smaller than the size of the copper pads:

qfn24-stencil-is-tcream

Note that the dimensions of tStop > Top > tCream, which makes sense. tCream is the smallest, because you usually actually don’t want your solder going all the way to the edge of the exposed copper. You get short circuits that way.

The tCream Stencils Are Usually Pretty Messed Up

One huge issue encountered quite frequently when using SMD parts in EAGLE is that the tCream pad sizes simply put too much solder paste onto the board.

tcream-equals-pad-size

People need to remember to adjust their device Packages and shrink the tCream rectangles, particularly for large pads that could cover a lot of paste and cause shorts under the package.

The default SMD part in EAGLE has tCream covering the entire pad, which will probably be a problem.

You can use the Design Rule Check -> Masks -> Cream setting to shrink the size of the stencil by some amount, but it’s not clear that this is the most effective way to design the stencil. It may require more specific, per-Package tuning. Using the DRC setting applies the rules to all SMD devices in the design.

drc-cream-change

But this doesn’t make sense, for instance, for simple devices like a 0805 resistor or capacitor, as having a lot of solder most likely will not cause a reflow short to occur.

And using these settings may cause problems for certain 3-terminal SMD chips.

Grid Units and Size

When creating surface-mount part packages, go to the command-entry bar (which might be in inches):

EAGLE CAD Grid Default

And type:

grid mm; grid alt mm;

And the system will then be thinking in millimeters:

EAGLE CAD Grid Default (mm)

You might want to set the grid to match the grid pattern in the datasheet. Let’s say you’re creating a VQFN12 part, with a 0.5mm pitch:

Land Pattern VQFN12

Then you can see that each pin center is offset by 0.5mm. So you can tell EAGLE CAD to set up a grid and alt grid this way by saying:

grid mm 1; grid alt mm 0.5;

Through-Hole Pads

When creating a through-hole via / pad it doesn’t automatically create a tStop mask that can be easily soldered to. I’ve gotten bit by this. It will create a plated through-hole, but not a copper pad. This is evil.

eBay Seller Bait and Switch Maneuvers

So here’s a little story about eBay, inspired by my recent attempts to buy a used laptop, from a (likely, highly unreputable) seller. I will name names.

I spent a good part of Sunday this week trolling eBay, searching for the best deal on a used laptop for some upcoming programming projects I have in mind. After researching and researching, digging up Thinkpad Personal Systems Reference books, and all sorts of other Manufacturer parts specs, I bid on a Dell from a user named “terbusinesses“.

The bid was on item 200884952022, which was initially a Buy It Now auction with a $350 starting bid and a $400 Buy It Now option. So, at 12:26pm Central Time, I bid $350. And I received a confirmation notice from eBay that I was the High Bidder. It looked like this:

Confirmation e-mail from eBay, that I opened the bidding on item 200884952022.
Confirmation e-mail from eBay, that I opened the bidding on item 200884952022.

For those in the know, once a bid has been placed on a Buy It Now auction, the actual option to Buy It Now goes away, the auction becomes a normal highest-bidder-wins type of auction.

A few hours later, I’m browsing eBay, and notice that “terbusinesses” has listed the identical item that I’m bidding on under a second auction, item 200886900878, which is the same sort of Buy It Now auction with a $350 starting bid with a $400 Buy It Now option. I don’t have a screenshot of this, but I know what I saw.

A few hours later, I get an e-mail: “eBay Bid Cancellation Notice – Item 200884952022 : DELL LATITUDE E6420 LAPTOP MOUSE, 4GB, CORE i5 WIN 7 INTEL HD 2.5 GHZ 320 GB HDD”. It looks like this:

The notice eBay sent me after the buyer rigged a second auction with the same item.
The notice eBay sent me after the buyer rigged a second auction with the same item.

That’s interesting, I think.

So somehow, this seller/scammer is hooking people into Buy It Now deals, and cancelling previous in-progress auctions. I’m not sure how this works on eBay’s side, because I don’t believe it should be possible to prematurely end a normal in-progress auction. And it certainly shouldn’t be legit to list the same item twice, then accept only the auction result that you want. In any case, this certainly doesn’t do much to build my confidence that the platform is fair, if this is a widespread thing.

The stupid bullshit reason that eBay gives when I look at the original item I bid on is: “This listing was ended by the seller because the item is no longer available.”, which is crazy. Tell me that at 12pm today there were “2” of the same item being sold by someone, and now there are none?

What eBay says when someone bait and switches an auction on you.
What eBay says when someone bait and switches an auction on you.