Working With A Manufacturer - The Best 0402 Footprint

April 24, 2015 Chris Denney

So you've got a great idea and you want to bring it to market, but you don't know the first thing about hiring a manufacturer and working with them to get your product assembled? We are here to help. This is one piece in a series of articles that we'll be writing that will help people learn how to work with us to get their product assembled.

We see a lot of different, unique assemblies come through our shop. On average we are introducing a new job to our factory every single day. This means the assembly has never been built by us and likely has never been built by anybody. With this amount of new work, we've seen the good, the bad, and the downright ugly when it comes to footprints.

The most sinister of all footprints is the 0402 footprint. There's something about the 0402 component that just loves to drive us insane! The issue is that many footprints inadvertently cause the component to "tombstone" or worse "head-in-pillow". Here's what those defects look like.

This is what a "Head-In-Pillow" defect looks like. Notice how the right side isn't actually wetted to the component.

This is what a "Tombstone" defect looks like. It might be hard to notice at first, but the pad on the left has the 0402 capacitor standing straight up in the air.

The biggest culprit of all 0402 issues is the default 0402 footprint in the Eagle library that comes with every installation of Eagle (I love Eagle. I really do. I just hate this one footprint). So, if you have 0402 components in your design, please, for the love of all that is honest and good in this world, change that 0402 footprint to the one we recommend.

Here's a video showing me editing this footprint in Eagle.

The perfect 0402 Footprint

This video was made using the Eagle EDA tool. But here are the dimensions you'd want to use if you designed this in any other EDA tool.

We have populated hundreds of thousands (probably millions) of 0402's with this footprint. We've had such success with this footprint that I had to reach out to the engineer who designed this footprint originally and thank him for his design. He thought I was ridiculous because it was just the standard dimensions that he found on a datasheet many years ago. But the difference is he actually created his from scratch, rather than relying on the default footprint in his EDA tool's library. And this is the key. Whenever you start using a new footprint, please check it against a datasheet and make sure it matches. Your manufacturer will thank you.

If you ever have any questions, please feel free to reach out to us. We'd love to hear from you. My email is address is cdenney@worthingtonassembly.com and our phone number is (413) 624-6879.

Working With A Manufacturer - What Are Fiducials and Why Are They Useful?

April 17, 2015 Chris Denney

We get this question all of the time. Many designers have no idea why they would put some random piece of copper on their board when it serves them absolutely no purpose.

Fiducials are reference points on the PCB for automated equipment. Its full name is "Fiducial Marker" and according to Wikipedia a Fiducial Marker is

“an object placed in the field of view of an imaging system which appears in the image produced, for use as a point of reference or a measure.”

In other words, fiducials help machines recognize where an object is in its space. For PCB assembly this means that our stencil printing machine, pick and place machine, and AOI machine can recognize where the PCB is when it goes to perform its task.

We encourage customers to use 3 fiducials. If you can't fit 3 fiducials then 2 fiducials will generally suffice. But 3 fiducials is the best. Each one serves a purpose.

1st - The first fiducials helps the machine recognize where the PCB is in its space in the X and Y dimensions.

2nd - The second fiducial helps the machine recognize what orientation the PCB is in and also how skewed the PCB is in the clamps. If the PCB is rotated even 1/10th of a degree this could completely ruin the assembly if it weren't for fiducials. The machine is able to measure the angle that the board is rotated in the machine down to the nearest 1/100th of a degree and compensate all of the placements accordingly.

3rd - Finally, the third fiducials helps the machine compensate for any shrink or stretch of the PCB. Yes, believe it or not PCB's do vary by very small amounts over a long enough distance. This is very important for larger PCB's because they experience a much greater amount of stretch and/or shrink. This is especially true for double-sided SMT assemblies. After the first side is reflowed in the oven the board may have stretched, shrunk, bowed, flexed, whatever you want to call it. Having the third fiducials can help compensate for this effect.

The next question people ask is, well why stop at 3? Why not have a 4th? Well, imagine this scenario. Your PCB is placed inside the machine rotated 180 degrees. What happens when the machine goes to inspect the 3 fiducials? It will find your 4th fiducial and start populating the entire PCB thinking that it understands it orientation correctly. This has happened to us, more than once, by well meaning designers. So, for the sake of sanity and my ever growing collection of gray hairs, please only put 3 fiducials on the PCB.

Fiducials are necessary whenever SMT components are going to be placed onto a circuit board. This includes double-sided SMT assemblies. So make sure to put fiducials on both sides of the PCB because cameras can't see through circuit boards. It needs those fiducials on both sides.

So when you are designing your next PCB or revising and existing PCB, please consider adding fiducials to your design. Many designers have found that using a 1mm round fiducial with a 2mm masking area around it works best. This masking area eliminates any glare that might reflect into the camera from the glossy finish of the masking. The best placement of these fiducials would be at the corners of your PCB. Not all the way up to the edge. This could cause the clamps of the machines to cover the fiducials. Try keeping them about 5mm from the edge or so. You can probably squeeze them in as tightly as 3mm but that's cutting it close. 5mm leaves plenty of breathing room.

We have put together a nice document about automated PCB handling. You can download it by clicking here. Also check out our best practices page if you haven't already done so for other helpful information.

As always, if you have any questions, please do not hesitate to reach out to us. We'd love to hear from you. You can email me at cdenney@worthingtonassembly.com or give us a call at (413) 397-8260.

Working With A Manufacturer - What's a BOM?

April 13, 2015 Chris Denney

So you've got a great idea and you want to bring it to market, but you don't know the first thing about hiring a manufacturer and working with them to get your product assembled? We are here to help. This is the first in a series of articles that we'll be writing that will help people learn how to work with us to get their product assembled.

So, what's a BOM? Well, you probably already know what a BOM is (Bill of Materials) but what are we specifically looking for from a BOM.

There are 3 vital pieces of information.

1. Reference Designator - this will be the location of your component. Something like R1 for resistors or U1 for IC's. You begin with a prefix and then assign them a unique number. Here are the common prefixes

Capactiors "C"
Connectors "J"
Diodes "D"
Displays "DISP"
Ferrite Beads "FB"
IC's "U"
Inductors "L"
LED's "LED"
Modules "U"
Oscillators (Crystals) "Y" or "X"
Resistors "R"
Resistor Networks "RN"
Switches "SW"
Test Points "TP"
Transformers "T"
Transistors "Q"

2. Manufacturer's Part Number - we'll need to know what part to put in the location you've specified. And what we are looking for is a specific manufacture's part number. Saying "100k ohm resistor" isn't quite enough information. Think for a second how many pieces of information need to be specified for just a resistor. Value, tolerance, size, wattage, composition, temperature coefficienct, and operating temperature. And that's just for resistors! It's just too much information than we'd like to decide for you. But an excellent resource is the Common Parts Library which WAi helped assemble in partnership with Octopart. We regularly keep these parts in stock and find them to be readily available and inexpensive compared to their alternatives.

3. Quantity - this is helpful for purchasing purposes. If you used a 1K resistor in 90 different locations, we'd rather not count those locations one by one to determine how many of that part number to buy. The quantity column will also help us double check our work. If we program our equipment and it tells us that there are 89 locations and you have a quantity of 90 specified, that will send up a red flag that there might be an issue.

There's plenty more information that some of our customers put in their BOM that are useful but inessential.

4. Line Items - this is handy for communicating back and forth. If we mention "There's a typo on line item 16" you'll know exactly where to look at your BOM for the issue.

5. Descriptions - Descriptions are really helpful. Internally, we use Digikey's descriptions as much as possible because it's formatted nicely and has all of the pertinent information. We can also use the description to make sure that your manufacturer's part number is correct. If your description says that it should be a 4.7k resistor and when we look up the manufacturer's part number and find that it's a 47k resistor, we can check with you to make sure you've chosen the correct part number. You'd be surprised how many mistakes we catch with this technique.

6. Datasheets - it's pretty useful to have a hyperlink to datasheets for components. Sometimes manufacturer's don't publicly release their datasheets (I'm looking at you Broadcom!) So if you provide us a link to datasheets it helps us speed up the process of programming your board. But don't stress about it. It's not necessary.

7. Hardware Details - providing a nice full description of a piece of off the shelf hardware is great. If you give us a part number from McMaster-Carr for a hex nut, that's great. But a good full description of the nut is even more valuable. Is it stainless steel, or galvanized? Is it 3/16", how many turns per inch, etc. When we have a nice full description we can confidently buy these parts from an alternative manufacturer. I love McMaster-Carr (seriously. I love McMaster-Carr) but they're expensive. And we can find hardware for far less money from other suppliers.

When you're done, here's what a BOM will more or less look like. They're generally formatted in a table that can be parsed by Excel or another spreadsheet program. CSV files and tab separated files work great too. We'd honestly prefer them over a proprietary format like Excel or OpenOffice.

BOM's are vital. Will live and die by our customer's BOM. If you have any questions we'd love to hear from you. You can email me at cdenney@worthingtonassembly.com or give us a call at (413) 397-8260.