In industrial prototyping, we apply engineering principals to new designs in order to avoid errors.  This includes use of advanced math and computational software, peer review, and automated design rule checking just to name a few.

But even the best-laid plans can go awry after the design work is over.  Recently, I found myself in that exact situation.

Project Background

The design shown here is a part of an ongoing embedded systems project.  On completion, the printed circuit board assembly (PCBA) features copper footprints for dozens of surface-mounted components.  The individual resistors, capacitors, and semiconductors can then be soldered in place by hand or through automation.

Prototype Build

After performing the schematic capture and printed circuit board (PCB) layout, it was time to generate a bill of materials (BOM) and then order parts. 

Days later, the parts arrived in good condition like normal.  There was plenty of packaging to filter through, but overall there was nothing missing and nothing extra.

One by one, I started soldering the components into place.  When all was said and done, there were two errors in the entire layout:

• The first was a floating ground pin for a microcontroller.  The layout software had tried to warn me about it, but the net did seem like it was connected at the time.  This was not the case.  But minor mistakes like this are what engineering change orders (ECOs) are for.
• The second error was harder to deal with…

The problem involved an 8-pin level shifter chip.  Normally, soldering one into place would be a simple matter of pre-soldering one copper pad to use as an anchor, soldering the chip onto the pad, and then carefully aligning all other pins before soldering those in place as well.

But this time, the chip seemed to be too big for its own copper footprint.  What happened?

The galling thing was it wasn’t oversized by a wide margin—it was a difference of under 200 micrometers.  But at these length scales it was far too large a difference to “shoehorn” in without risking a short circuit.  This was clearly either the wrong part, the wrong footprint, or possibly the wrong part and footprint.

Isolating the Mistake

In the real world: there is the part in the design, the part on the bill of materials, and the part that physically arrives on the delivery truck.  All 3 are not necessarily the same.

After checking my own documentation against the manufacturer’s drawings, my mistake became clear:

The device on the bill of materials is the Texas Instruments, ‎SN74LVC2T45DCTR‎, marked as an “SM8” (SSOP).  However, its base part number is SN74LVC2T (or effectively: SN74LVC2T45).  The base SN74LVC2T45 family comes in 3 packages according to the official website:

• The 8-pin Die-Size Ball Grid Array (DSBGA) (with part number code: “YZP”).  This style is a ball grid array that can’t be hand-soldered.
• The 8-pin Very Thin Shrink Small Outline Package (VSSOP) (with part number code: “DCU”), which was the one I should have ordered.
• And the 8-pin shrink small-outline package SSOP (“DCT”), which was what I actually ordered.

In short, the SSOP part I had in-hand had more space between its pins (pitch) than the footprint would allow.  In this case, it had a 0.65 mm pitch compared to the intended 0.50 mm.

Summary

My own bill of materials showed that I had accidentally selected the SSOP variant for purchasing.  The part description simply describes the devices as “SM8,” which could easily stand for “surface-mount 8” and refer to either footprint.

Luckily, this part was not expensive.  The few that I had ordered can always be reused later.  At under 1 USD apiece, fixing the problem was simply a matter of ordering the correct part and revising the BOM.  But it just goes to show that it pays to double-check package names when going to buy parts.

For More Lessons Learned

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