Category Archives: Screaming Circuits Blog

EMS provider Screaming Circuits offers tips and thoughts on design and manufacturing

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/

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

Never Take Pin Numbering for Granted

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Our all-things-about-electronics manufacturing standards body, the IPC, specifies the proper numbering order for most components. That’s a pretty nice thing that they do there, but it’s not always enough to prevent layout mishaps. Case in point a line of small PCB mount switches.

IPC calls out pin numbering for dual inline components, with pin one on the upper left (at zero degrees rotation), counting down, then over to the bottom right, and counting back up, as in the illustration below.

Given, that, it would be logical to assume that all dual inline components follow the same pattern. Logical, yes. Accurate, no. Multi-color LEDs, connectors and switches are some of the more common offenders.

In this particular switch, it’s not just a case of the numbering not following convention, it’s also different from one variant to another. I understand why. The switch isn’t changed between through-hole, top mount surface mount and side mount surface mount, but the leads have to be accessible from different parts of the package.

The following two footprints are from the same switch. One mounts on its side, and the other, standing up.

The pin one numbering doesn’t follow convention, nor does the numbering of pins 4 – 6. And, you may have also noticed that the two are top-to-bottom mirror images of each other. Ugh.

This is why my mantra is: Always check the datasheet. Always.

Duane Benson
Don’t take it for granite either, because granite is too heavy.

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 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

 

Let’s Talk about HAL – For Big Parts Only

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The board surface names: HAL and HASL (hot air leveling and hot air surface leveling) refer to the same thing. They are interchangeable terms. With that out of the way, I’ll get to my point, which is that HASL is not the right surface for all applications.

Take a look at the photo on the right. This is a 0.5 mm pitch BGA land, using lead-free HASL. Don’t expect good results with this board. It’s a good quality HASL board. Even the bumps on the pads are not out of line for a HASL PC board. It’s not a defect. It’s the HASL works.

The catch is that, while the PC board is perfectly good, it’s not the correct board surface to use for all parts. HASL is fine for larger parts, but for small components, it’s archaic and not reliable.

BGAs require a flat surface (also called a planar surface). With the bumps common on HASL boards, the BGA won’t have a flat surface. The solder paste won’t adhere evenly to the pads. The BGA will probably slide off the pads before reflow. It may end up far enough off that it can’t self-center, as BGAs usually do.

The HASL pads won’t all have an even amount of solder left on the board. Some pads will have more, some less. When added to the solder paste, the pads with more solder may end up bridging.

All of the issues become even more severe as the parts get smaller. Wafer scale parts, 0.4 mm pitch parts, 0201 passives, and other similarly or smaller sized components are essentially incompatible with the HASL surface.

So, what do you do? Order your boards with immersion silver or ENIG. Both give a nice flat surface that BGAs like.

Duane Benson
Open thse Posd Basy Doors Hasl

http://blog.screamingcircuits.com

Milling Madness

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Sometimes, we find things that kind of defy explanation. Fortunately, this didn’t come from Sunstone, our normal board house.

Regardless of who it came from, I’m sure it was a one-off mistake, but, wow. How could anyone miss this?

 

 

 

 

 

 

 

 

 

It just goes to show, it’s always a good idea to take a look at what you get from your board house before sending it on to us.

Duane Benson
Termites, maybe?

http://blog.screamingcircuits.com

Designing for Movement

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What is the difference between electronics in a robot vs., say, a stationary temperature monitor and control device? For one, if the temperature controller goes haywire, you can pull it off the wall and stomp on it, while you might have to chase the robot (or be chased) to deactivate it if it’s gone into world domination mode. More relevant, though, is vibration.

Fixed embedded electronics generally don’t need to worry about vibration induced reliability issues. Mobile robots, however, do. Unsecured connectors can work their way loose. Bolts can back off. wires can brush against stuff. A lot of practices that don’t cause problems in a fixed installation can bite in a mobile setting.

For example, a simple board-to-board ribbon cable. On the left is a common friction-retention cable connector. Fine for a development board, but not for an environment subject to vibration. Instead, use a mechanically captive connector, as shown on the right.

 

 

 

 

Free hanging cables are also a “no” for mobile devices. Cables hanging loose can get caught on edges, or tall or hot components. That can lead to worn or melted insulation and shorts. Instead, use cable ties, insulating grommets, and careful routing.

There are plenty of other considerations, but these are two of the biggest traps to avoid when movement is called for.

Duane Benson
Klaatu barada nikto. Translation: “Spaceman says what”