Tag Archives: Duane Benson

Electronics Manufacturing Files: What We Need

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Manufacturing is all about taking data from you and delivering some good working circuit boards. Well, it can be just data — as in full turnkey — or data plus some parts and or PCBs, as in a partial turnkey or a kitted job. Regardless of whether you’re sending parts and boards, or having the us as the EMS buy everything, we need good data, and a lot of it.

That data are the difference between the working boards you want and need and a random jumble of expensive paperweights.

We need a bill of materials (BoM), the job specifications (which you give us by ordering and describing any special instructions on our website), and the CAD design files. Fab and assembly drawings are always a good idea too. A little extra time spent on the files sent reduces risk, and that’s a very good thing.

The CAD design files include Gerbers, a centroid (aka pick-and-place or XYRLS file), and intelligent CAD files, such as ODB++ or IPC-2581. In some cases, such as Eagle CAD, we can use the native CAD board file.

The ODB++ and IPC-2581 file formats are the future of electronics manufacturing. They come with more data, and more accurate data, than do Gerbers. If you can send either of these two, please do so. Even if you have those, still send us the Gerber files. Gerbers are the lowest common denominator, and provide a base that we and PCB fabricators can work from.


The Gerbers are a set of files used to create the various layers of the board. Each layer requires an individual file, so a six-layer board (six copper layers) will typically require at least 13 distinct files: one for each copper layer, top solder mask, bottom solder mask, top silkscreen, bottom silkscreen, the drills holes, and solder paste for the top, and bottom if the board has SMT parts on both sides.

The drill file is combined with the Gerber files to line up the via and through-hole component holes with the appropriate spots in the PCB. Then the pick-and-place file will tell the assembler where to put each component, what angle to place it at, and which side of the board it goes in.

Fab drawings hold a human-readable, often in PDF format, description of the board and any special instructions needed by the fabricator. The assembly drawing would be the same, but for the assembler.

Sometimes the parts are too densely packed for the reference designators and polarity marks to show up on the actual board, or for aesthetic reasons, the designer doesn’t want them on the board. In such cases, all that information would be put into a set of assembly drawings; PDF files showing all of the necessary reference information.

As of this writing, the ODB++ and IPC-2581 file formats aren’t universally accepted, but are getting more so all the time. Use of these new intelligent CAD output file formats helps to reduce the number of manual steps and human interpretation, and will eventually lead to better quality and faster manufacturing times.

Duane Benson
What do we need?
What does it really matter?
Matter converts to energy
E=(what we need)C2

Power Distribution – To Route, or to Plane PCBs

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Power distribution on a PCB can come in a number of forms. The three most common methods are:

  • Route power and ground.
  • Use surface layer floods.
  • Use internal planes.

After component positioning, you’ll need to look at power and ground distribution. With a two-layer board, your options are limited to individually routing power and ground, or using a polygon fill, also called a flood or pour.

 

 

 

 

For simple low-speed layouts, it’s common to route power just like any other signal. You’ll typically use a wider trace, which you can set manually, or with design rules. Drawing a polygon in the board shape, and giving it the same name as your power or ground signals may make the job easier. Keep in mind though, that you can end up with parts of a ground plane disconnected from the rest of the board. This is called an orphan. Some CAD error checks will spot such a problem and some won’t.

I made that mistake not long ago, as describe in this blog post.

If you have a four (or more) layer board, common practice is to designate one of the internal layers for ground, and one for power.

 

 

 

 

 

 

Doing so can leave more room for signal routing, can reduce EMI, and can leave a cleaner-looking, easier-to-debug board. It also reduces the chances of having orphan ground or power areas, as I warned against in the prior post.

Duane Benson
Chocolate layer cake with coconut frosting will not help with EMI

http://blog.screamingcircuits.com/

Basic Layout — Aligning Components

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Not long ago, I designed an Arduino compatible clock board. The board has 12 NeoPixel (digital addressed RGB LEDs) arranged around the board to act as hour hands. The minutes and seconds are represented by an external ring of 60 NeoPixels.

 

 

 

 

 

 

 

 

 

 

How did I go about positioning the 12 NeoPixels, and what does it matter? For aesthetic reasons, I do want each NeoPixel in the proper place. If any are off a bit, I’ll notice every time I look at the clock.

I created a triangle, with all of the correct distances, and drew in in my CAD software’s Document layer. The Document layer looks just like a silk screen layer, when visible, but it won’t be printed on the board. You can use this layer to put in extra information for yourself, or for the manufacturer.

 

 

 

 

 

 

 

 

 

 

You’ll notice that I also wrote in the document layer “No tabs here.” That’s an instruction to the board fabricator to not put a panel tab where the micro USB connector goes. If it did, the board wouldn’t be buildable when panelized.

Some create a fabrication document layer and an assembly document layer. An example might pertain to reference designators. If the board is too compact for reference designators, of if, for aesthetic reasons, you want to leave them off the finished board, You can put the reference designators in an Assembly Documentation layer. Then be sure to let your assembler know what you’ve done.

The other things I did here is to keep all the LEDs aligned with the baseline of the PCB. In theory, you can place a component at any rotation angle you want. But, like any system, manufacturing works better when there are fewer variables.

You reduce the probability of error if you keep components aligned at factors of 90 degrees. It also helps to keep polarities oriented the same way, as much as possible. For example, if you can, have all the diode polarities facing the same direction.

Duane Benson
Time flies like an arrow; fruit flies like a banana

http://blog.screamingcircuits.com

The Ideal Bill of Materials

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A good portion of a quality build is simply the result of clear information. One of the more important pieces of information we deal with is the bill of materials, called “the BoM.”

The BoM is a list of all the components to be placed on the PCB. The file typically includes an index number, the number of times a specific component will be used on the board, the reference designator from the schematic, the component manufacturer, and the manufacturer’s part number.

If a specific component is used more than once – a common bypass capacitor, for example – it will still only take up one line in the BoM. One field in the BoM will list the number of times the component is used, and another field will list all the reference designators for that part number.

For example, line 5 in my BOM on this slide, is a 0.1 microfarad, 10V capacitor.

The first field in the table has a line item index, 5, because this is the fifth unique part number in my BoM. The next field has a quantity of this component used on the board, which is 5. Field three holds reference designators C1, C2, C3, C4 and C5. The next field has the manufacturer, and the final field has the manufacturer’s part number.

You will likely have additional fields, such as a distributor part number, a description, the package type and other tidbits, as I have here.

But the first five columns in this example show what is generally considered to be the minimum data set for a good bill of materials.

Note the three lines at the bottom highlighted in red with the label “DNS” in the Type column.

DNS means “do not stuff.” That’s an instruction to the manufacturer to not install that component during the assembly phase. Some people use DNP, for do not place, or DNI, for do not insert. It’s always best to consult with your manufacturer to get their preferred labeling.

You may also want to include alternate parts for components likely to go out of stock. Passives, such as capacitors and resistors, are notorious for going out of stock without notice. Invariably, though, a half dozen nearly identical parts will fit the bill just as well.

Create an alternates list so the purchasing folks or manufacturer won’t get stuck not knowing if a substitute is valid or not.

Duane Benson
In the 90’s, when people said good things were “the bom”, this is what they were talking about

 

Mistakes Were Made — Too Much Ground Isolation

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I recently ran a batch of my Neo Pixel clock boards through the factory here. It’s an Arduino UNO-based design that I made for myself not long ago. It sports an Atmega328P, with bootloader, an FT231X USB chip, and a DS3231 real time clock (RTC) chip. Pretty standard stuff. It doesn’t even use small parts. All the passives are 0805 size. There’s nothing exotic here. So, where did I go wrong?

I also used my 3D printer to make a clock frame to hold this board and a 60-pixel ring of NeoPixels, from Adafruit. I found that with the micro USB connector on the top of the board, it’s a little awkward to plug in the USB cable, so I put pads for the connector on the back side of the board. Depending on exactly where and how the board will be used, the micro-USB, button switches, and clock backup battery can all go on either the front or back surface of the board.

Programming the bootloader worked as expected, so I assumed it was just a job well done. Except it wasn’t. When I plugged in the micro USB cable, the RX and TX LEDs flickered briefly, but the board wasn’t recognized by my PC.

Take a look at the back side of the PCB and see if you can find my mistake (spoilers after the photo).

I ran a 24 mil trace around the back side of the board to supply power to the NeoPixels. That’s not a problem, except that I closed the loop on that trace, and didn’t put a path for the ground to get across the trace.

Follow it around, and notice that the ground connections to the u-USB connector don’t go anywhere except to this part of the plane. Ugh.

Duane Benson
Cassini’s gone now.

QFN Center Pad Revisited

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The QFN (quad flat pack, no leads) package can no longer be considered exotic. It was when I first wrote about it a decade ago, but not anymore. In fact, with the wafer-scale BGA, it’s one of the more common packages for new chip designs.

Not all QFNs come with an exposed metal pad underneath, but most do, and that can cause problems with reflow solder. The pad itself isn’t the problem, but improper solder paste stencil layer design can be.

The default stencil layer in the CAD library footprint might have an opening the full size of the metal pad. If that’s the case, modify the footprint so that there will be 50% to 75% coverage with solder paste (Figure 1). If you don’t, it may result in yield problems. With a 100% open area, the likely result is too much solder in the middle. The part will ride up, or float, and may not connect with all of the pads on the sides of the part.

Figure 1

Figure 1. The optimal QFN footprint will have 50% to 75% solder paste coverage.

 

Figure 2 shows a stencil with too large an opening in the center, a segmented paste layer in the CAD footprint, and the resultant segmented stencil.

Figure 2

Figure 2. Stencils shown with too large an opening in the center (left), segmented paste layer (center), and the resultant segmented stencil (right).

 

You may note that I said to shoot for 50% to 75% coverage and ask: “Well, is it 50% or 75%? What gives?”

True, that is a bit of ambiguity. Anything in that range should be fine for prototype boards, however. If the assembly is headed for volume production, work with the manufacturer to tweak the design for best high-volume yield.

The good news on this front is that many QFN manufacturers and parts library creators have taken notice. It’s far more likely now than it was 10 years ago to find a datasheet correctly illustrating this, and footprints created correctly. But, always check your footprints to make sure.

Duane Benson

http://blog.screamingcircuits.com

Components So Fragile, They Break Before Arrival

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There are a lot of components that require special handling. Some days, “special” requirements seem more the norm than the exception. But, every now and then, we see something that puts even those special components to shame.

Not long ago, we received a parts kit that contained a component so fragile, that most of them didn’t survive the trip with the shipper. It’s a 10 x 9mm (well, actually 9.68 +0.00/- 0.08mm x 8.64 +0.00/- 0.08mm, to be precise) sensor that’s only 0.05mm thick. That’s 1/4 as thick as the diameter of the solder balls connecting it to the PCB.

The part has solder balls on the silicon, with no other packaging. The dice has to be that thin, as the light-sensitive area is on the other side. That doesn’t make for a very robust component. It would require special handling all around. Unfortunately, no matter how careful we might be, if they’re broken when we receive them, there’s not much we can do (other than take pretty pictures).

In taking these closeups, I noticed that the registration in ball placement isn’t all that great. In the image below, take a look at the ball on the left, second from the bottom, and the ball on the far right.

The datasheets call out all non-specified tolerances as +/-0.001mm. With these being 0.2mm diameter solder balls, I’d have to say this is way outside of that tolerance. I’m sure the part would have adhered to a decent board just fine, but if the PCB were off a similar amount in the opposite direction, you may very well have a problem.

 

Duane Benson
You could make a very tiny sundial out of this.
But, could you use the shadow parallax to calculate the distance to the sun?

http://blog.screamingcircuits.com

What’s So Difficult about Diodes?

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A diode can be put on a a PCB in one of two ways. It’s only got two pins (usually — see, I already have a caveat). I’ve written about them a few times before. I’ve got a sampling of those posts here. But first,

Good marking:

 

 

 

 

Bad marking:

 

 

 

 

The diode schematic symbol is always a good choice. If there isn’t room for that, “A” for anode or “K” for cathode work well too. Why “K”, and not “C”, you may ask? Because “K” kan’t be konfused with a capacitor.

Okay. Enough ranting for now. Just use the diode schematic symbol, “A”, for anode, or “K”, for cathode; and always look at the data sheet for the exact part number.

Duane Benson
1 cricket per chip

http://blog.screamingcircuits.com

What Makes a Good Fiducial?

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Accountants may have a fiduciary responsibility, but that really has nothing to do with PCB assembly. Change the “ry” to a “ls,” however, and you get fiducials, which does have something to do with PCB assembly.

A fiducial is essentially an alignment mark for surface-mount assembly machines. High-volume assembly requires them to ensure accurate registration and parts placement. Low-volume assembly, like we do at Screaming Circuits, doesn’t necessarily require them. (Some low-volume shops do, so ask before assuming.) Even if they aren’t required, they still help and are always a pretty decent idea.

The basic idea, explained in this blog post here, is to create a non-reversal pattern with two or three fiducial marks on the board or panel. As you can see in the image above, the designer placed three fiducials around the board in a non-reversible pattern. (To protect the confidentiality of the board design, I obscured the circuit detail with this convenient robot head.)

In terms of the specific construction of a fiducial, two things are most important: contrast, and accuracy of position.

Contrast comes from it being bare copper – make it 1 to 2mm in diameter. Don’t cover it with solder mask. Make the mask opening 2 to 5mm larger than the copper.

The image on the left shows closeup detail. This particular fiducial mark uses a square cutout in the silk screen. Most use a round cutout, but the shape isn’t all that important. The copper pad should be round, though.

Making it out of copper gives the positioning accuracy. I’ve been asked why silk screen markings aren’t acceptable. Silk screen isn’t always registered consistently, and is therefore won’t ensure accurate alignment. Don’t use silk screen as a fudicial or positioning mark of any kind.

Again, they’re generally required for high-volume manufacturing. We (Screaming Circuits) don’t require them for low-volume, but some assembly houses do. Even when not required, they’re still a good idea.

Duane Benson
Fiducial on the roof is a long movie
But at least it stays in place

http://blog.screamingcircuits.com

Let’s Talk about HAL – And Another Thing

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A few days ago, I wrote about HASL PC board surfaces, explaining that it’s not an appropriate choice for small parts.

Look at the same PCB image I used the other day. You might not recognize it because before it was on the right, and today it’s on the left. Getting past the fact that I just insulted everyone’s intelligence, there is something else about this board that we don’t recommend.

I’ll give you 30 seconds to figure it out. I don’t have a stopwatch, so the 30 seconds is on the honor system.

This is a land for a 0.5mm pitch BGA. As I wrote before, HASL is not the right choice for BGAs, especially for those of the smaller pitch variety. The other problem with this board is in the pad layout.

These are solder mask defined (SMD) pads – the solder mask covers the outer part of the pad, so the solderable copper surface is determined by the size of the opening in solder mask, not by the area of the copper pad.

For BGAs 0.5 mm pitch and larger, we (and pretty much everyone else) recommend non-solder mask defined (NSMD). With a NSMD pad, the solder mask opening is larger than the pad. This leaves more copper area to adhere to, including the sides of the copper pad. It tends to be much more reliable.

The image to the right illustrates the difference. 

The left-most pad in the image illustrates an SMD pad, while on the right is an NSMD pad. The NSMD pad leaves a lot more surface area of the copper pad for the solder ball to grip on, including the sides.

BGAs with 0.4mm pitches might need either SMD or NSMD pads, depending on a number of circumstances. Read this blog information for a bit more on 0.4mm. When in doubt, look in the back of the datasheet.

Duane Benson
Question for physicists and mathematicians:
Should the last recursion in the Mandelbrot set land on Plank’s constant?
Show your work.

http://blog.screamingcircuits.com