Screaming Circuits: Tips and tools


Cost Reduction in Design - More Advice for Makers

If you're looking for the absolute, cheapest possible assembly service, you'll need to look outside of North America. If you really need a decent price with good quality and good service, you can keep your gaze West of the Atlantic and East of the Pacific.

Like everything else in the modern world, design decisions can have a pretty big impact on your cost. So, lets take a look at some design decisions that can make your manufacturing more affordable.

  • Accept longer lead times

Lead times are one of the biggest factors in electronics manufacturing. Screaming Circuits can turn a kitted assembly job overnight, but it costs a lot of money to do that. Screaming Circuits also has a 20 day turn-around that is much, much more affordable. Accepting longer lead times on PCB fab will drop your cost as well.

  • Avoid leadless packages like QFNs and BGAs

We build tons of QFN and BGA boards - even down to 0.3 mm pitch micro BGAs. That's great if you need those packages. However, since all of the leads are underneath, we have to x-ray every part. That adds a bit of cost to the process. If you can, stick with TSSOPs and other parts with visible leads.

  • Use reels, or 12" or longer continuous strips

Tab routed multi panel 1024We will gladly assemble parts on strips of almost any size. But, to save costs, use full or partial reels or continuous strips of at least 12" long. It costs us less time to work with reels and continuous strips, and we pass those saving on.

  • Stick with surface mount

These days, thru-hole components tend to be hand soldered. That costs more than machine assembly, so use surface mount wherever possible. Surface mount components tend to be less expensive than thru-hole too. If you do need a few thru-hole parts, this is an opportunity to put in a little sweat equity by soldering the thru-hole yourself and save a bit of money.

  • Panelize small boards

We can work with really tiny boards individually, but sticking with a larger size makes the job easier, and, again, we'll pass those saving on. If your PC board is smaller than 16 square inches, panelize it. We put in less labor and you get a price break.

By sticking with Screaming Circuits, you get the same care and quality that we give to boards going up into space, down into the ocean, and everywhere in between. By sticking with Screaming Circuits, you get a known turn-time; not an "about ..."

By following these guidelines, you get a decent price and really good quality and service.

Duane Benson
That would be telling

Choose Your Package Wisely

As I mentioned in my prior blog, there are reasons to consider different packages than just physical size.

Sometimes it is just space available on the PC board, but there may be other considerations as well. One of the first to consider with really small size packages, is the capability of your manufacturer. Not all assembly service providers can TI ESD CSP 007 croppeddeal with super-duper small parts.

That's a paperclip next to the little ESD protection chip in the photo on the left. At Screaming Circuits, we can go down to 0201 passive parts and 0.4 mm pitch BGAs. We've even done a few 0.3mm pitch BGAs, but those are pretty rare still.

Some manufacturers stop at 0402 (or even 0603) parts. If that's the case with your manufacturer, then you'll need to eliminate sizes smaller than their limit (or find someone else to build the board).

Cost might also come into play. It probably won't be enough of a factor to worry about during prototyping, but it may be worth looking at for volume production. Sometimes the smaller form-factors add cost. Sometimes the part value you need may not even be available in the smallest packages.

TI TPS62601 front and backIf both cost and size are significant drivers, weigh the cost savings from reducing the PC board area against any additional cost with smaller packages.

Noise can factor into your package choice too - especially regarding bypass capacitors on high speed chips. You want your bypass capacitors as close to the power and ground pins as possible. The higher the speeds, the more important this is. Dropping your package size down to 0402 or 0201 can make it easier to put the caps closer to power and ground pins.

Duane Benson
You don't need to ask Alice because your parts aren't ten feet tall

QFN? QFP? QFwhat?

The QFN (quad flat pack, no leads) has become my favorite integrated circuit package. It's very compact, yet is easier to use than a micro BGA.

Micro BGAs of 0.5mm and smaller pitch become a bit more difficult and costly with more than two rows of pins. At those geometries, escape routing can involve plugged and plated vias which adds complexity and cost to the board fab. QFNs can be almost as small, but have all of the pins exposed around the edges - so, no need for escape routing.

One thing that's important to note, is that despite sharing the first two letters (Q and F), the QFP and QFN footprints are not interchangeable. We do, from time to time, see boards laid out for one along with the other form packaged part. Arduino w QFN and QFP

Take a look at this PCB layout clip from the Arduino Leonardo. It has both footprints on the board. You can see how much bigger the QFP package is.

They put down both footprints because the Atmega32U4 chip used in the Leonardo sometimes has supply issues in one package or the other. This gives them the flexibility to use either without making any changes on the board.

You might consider this as an option if you have the space for a QFP and are concerned about the available of one package variant or the other.

If you do, there are some very important things to check out:

  • Make sure the pin-outs match. Some parts will vary the pin-out a bit between packages or have extra pins on one or the other.
  • Make sure the extra space won't cause noise problems. Generally, you want bypass caps as close as possible to the supply pins. This amount of extra space probably won't be a problem when using a QFN, but in some designs, it might.
  • Make sure the board won't be in an environment where unsoldered pads will be a problem. Some harsh environments could attack the unsoldered pads. If that's the case, consider conformal coat.

Duane Benson
We're always being pushed and shoved by people trying to beat the clock
But we like it - it's what we do

Warped PC boards

So... You just got a nice big PC board back from the fab shop. You set one on your desk to admire only to discover that it's warped. What do you do?

There are two primary types of causes of board warping: process related at the fab or assembly shop, and layout related issues. If it's warped before assembly, it's between fab and layout. If it's flat before assembly and warped, after, it's most likely between layout and assembly - although, sometimes a fab problem won't show up until a pass through the reflow oven at your assembly partner.

Determining the root cause is generally a bit of an iterative process. It's tempting to start right off with your fab or assembly partner, but you need some information before giving them a call. You'll need such things as the amount of warpage per inch, board size, and thickness. With that, you need to take a good look at your design and consider copper pours, component size, and component placement.

With that information in hand, you can make your phone call. If the board is warped before assembly, call your fab shop. If it's flat pre-assembly and warped post assembly, call your assembly house.

The shop you call will want to talk over your design to help you pinpoint the cause. If you can rule out a design issue,then you need to talk with your partner to determine whether it's a fab or assembly issue and next steps to take care of you.

 Here are a few design issues that could contribute to warping:

  • Uneven copper pour. Copper and FR4 are a good match relative to thermal expansion, but they aren't exact. A large pour on one side or corner of your board can lead to warping due to dissimilar expansion characteristics. This could cause warpage either at the fab shop or the assembly house.
  • Components with large thermal mass grouped together on the board. This would be more likely to cause problems during assembly than during fab. The thermal mass will act as a heat sink for that area on the board, which can lead to uneven expansion and uneven soldering.
  • A board that's too thin for the size or number of components could lead to warping at any stage.
  • Odd shapes or large cut-outs could also lead to warping at any point.

There may be other, more obscure causes, but those are the main design related causes. If it's none of those, talk with your partner.

Occasionally, design requirements lead to a board that is essentially non-manufacturable. Hopefully, you never have this situation, but if you do, make sure that thickness, component location, pours, or cut outs really, really, really, need to be the way they are.

If you absolutely, positively can't change anything, go back and try again. Then you can to look for heroic means to get the board fabbed and built.

Slight warpage might go away when the board is mounted. Just be careful with that. Some components may not stay securely soldered when you flatten it.

The board may need a special fixture during assembly to prevent warping. This will likely cost extra, but if you can't change your design, and still need it built, it may be your best option.

Finally, if nothing works, you may need to look harder at the design, or look for a new fab or assembly house. We all like to think we can do just about anything, but every shop has its limits, and on rare occasion those limits can be difficult to spot.

Duane Benson
What if Godot was late because he was waiting for John Galt?

Panel Rails - What Are They?

I referred to "panel rails" in my blog about V-score panels, but I didn't explain the "whats" and whys" of panel rails. You might find yourself asking "what are panel rails and why would I want to use them?"

Well, first of all, for our Full-Proto service, we don't require panels or panel rails. We'll take just about any old board that's bigger than 0.75" x 0.75" and smaller than 14.5" x 19.5" and run it through our machines. For our short-run production service, we only require panelization for boards less than 16" sq.

That being said, panel rails do have a purpose. They give the machines a spot to grab onto without coming close to components. They're also a convenient place to put fiducials (more fiducial info here).

As you can see in the image below, the panels give a clear area for handling the panel.

Tab routed panel

There are two important things to note about this panel. First, look closely at the four outside corners. You can see the scoring for easy separation of the rails. This designer made sure that there isn't any copper where the scores are. That's the right way to do it. The V-Score blog shows a panel rail done the wrong way - with copper across the cut.

Next, this board has fiducials. Good. But, the fiducials are in a symmetrical pattern. Not so good. IPC-7351b-3-10 specifies a non-symmetrical pattern so that the board can only be processed in one orientation.

Duane Benson
Once I build a panel rail, now it's done
Brother can you spare a diode

Individual routed boards - not panelized

Some people, especially in the manufacturing industry, refer to PC board panels by the term "palette." I can't seem to avoid thinking of the big wooden thing used for shipping stuff, so it's tough for me to call a panel a palette. It is, however, a correct designation - as is "panel.".

Milled cornerThis post is about individual routed boards, as in not in panels. Last time, it was tab routed, and before that, V-score.

On the right, you can see the relatively smooth edges of an individually routed board.

If panelization is so cool, you might ask "why not always panelize?" For large quantities, or really, really tiny boards, you really should ask that question because it's pretty much always a good idea. There are, however, good reasons not to panelize when in the prototype world.

First, with small quantities, you may not need enough boards to fill up a full panel. You can save quite a bit of money when ordering five individual boards, than if you had to order a panel of 30.

Fab houses tend to gang up board designs from a lot of different customers onto one panel. That allows for less waste and faster Milled edge 2turns for small quantity boards. The end result of that is that many fab companies charge more for panelization when quantity is small.

Our Full-Proto service can take individually routed PC boards down to 0.75" x 0.75". Our higher volume, more economical service, Short-Run requires that PC boards smaller than 16 square inches be panelized.

Duane Benson
Well my buddy Jim Bass he's a-workin' pumpin gas
And he makes two fifty for an hour
That's not very much

Tab Routing panelization

In my prior post, I covered V-sore panelization. The other very common panelization method is called tab-routing, as in routed, but with tabs. (That's "routed" like using a router, not as in Napoleon being chased out of Russia.) Following this paragraph, we have a tab-routed panel. I've obscured the detail of the PCB to protect the innocent.

Tab routed multi panel 1024

You can get it without the perforations, but if you're separating them yourself, you'll most likely be glad to have the perfs there. If we deem that snapping will cause undue stress on the board, we use a special tool to avoid putting that stress on the boards. If 1-Image30you're separating them manually, the perferations can make a big difference. Next, on the right, is a close up of a actual tab. The three holes make it "Tab Routed with Perforations."

A big advantage to tab routing is the ability to make boards in shapes other than rectangles. On the down side, it takes a bit more PCB material and can put a lot of stress on the area near the tab. That being the case, we recommend that you not put components too close to the tabs.

Now, the definition of "too close" is an interesting one. The IPC doesn't seem to have a specific standard covering the subject. 100 mils, or therabout's, is a reasonable target. Larger or stiffer parts might require a little more space.

When you purchase your PCBs in panels, you can separate them before assembly or after. Generally, the reason for panelization is for ease of assembly, so post assembly is the most common approach. Post assembly separation also requires the most care.

As I said, we have a special tool to avoid stressing the boards when nesessary. If you're separating them and don't have a tool, resist the temptation to just snap them apart like a Saltine cracker. Take some time and do your best to avoid much bending.

If snapped carelessly, or if parts are too close to the tabs, parts can break off. Sometimes the solder joint will just crack, leading to intermittent problems or later field failures. Use of some sort of cutting instrument that won't bend the boards is the preferred method.

Duane Benson
Have no fear; Underdog is here!

V-Score panelization

V-score top viewMy last post talked a bit about panelization, in general. Today, I'm taking a look at V-Score panelization. V-score is created by running a V-shaped blade across the top and bottom of the panel without cutting all the way through. The board in the mini-image of my prior post is V-scored. Top left, on this page, is a close up of the V-scoring. [Note that the cross-hatched area is not in the active circuit portion of the panel. It's in the rails. You'd never want to cut through copper like that in part of the board that will be used. Even here, it would be best not to have copper in the path of the v-scoring blade.]

You'll note that it's all straight lines. V-score can only separate rectangular panelized boards. For curves, you'll need to use a different technique.

V-score edge onThe next image down, on the left, shows an edge-on view of the V-score. You can clearly see what I mean by "without cutting all the way through." The cut leaves enough material to hold the boards solidly together during processing, but easy to separate.

V-score de paneled edgeBy the way, we generally don't just snap them apart. We've got a special tool - a bit like a pizza cutter in a fixture - specifically designed to separate them without stressing or bending the board. If we feel there's any risk of over-stressing, we'll use the tool.

The next image, here on the right, shows a board edge after de-panelization. Note that it's not a smooth, flat edge.

In contrast, the next image down, on the right, shows a flat milled edge. Generally, though, you can't visually tell the difference without close examination. You can, however, feel it if you run your finger lightly along the edge. Just be careful to not get slivers.

Next time, I'll examine tab-routing, which will allow for non-rectangular shapes.

Milled edgeDuane Benson
"I saw two Buffalos, two Buffalos,
Buffaloes on my lawn,
Romping all around and stomping on the ground
And all of my grass was gone."

PCB Panel Routing Technique

Most PCBs we receive are individually routed, i.e., not panelized. That doesn't mean that, sometimes, sending them in a panel isn't a good idea, or required. Generally, we don't require panels (sometimes called a pallet), but there are some cases when we do.

V-score panelIf the individual PC board, destined for Full Proto service, is smaller than 0.75" x 0.75", it needs to be panelized. If a PC board needing Short Run production service is less than 16 square inches, it needs to be in a panel of at least 16 square inches to qualify for Short Run.

So... you ask... why else might I want to panelize my PC boards? Keep reading and I'll tell you why.

  • First, if you've got a lot of small boards, it's easier to handle and protect then when they're in a panel. A few panels can be more safely packed coming and going from our shop here.
  • You may be able to get the through our factory faster. If you have a really large number, and need them super fast, panelizing them may enable that fast turn. With a lot of boards, sometimes, it simply isn't physically possible to put them all on the machine, run them and take them off, in a short turn time. Panelize them and the machine will be running longer for each board change, which reduces the total run time.
  • It may also cost you less. If you use leadless parts like BGAs, QFNs or LGAs, you can usually reduce your cost a bit by panelizing the boards. Leadless parts cost a little extra because of the X-Ray test needed, but the extra handling is mostly per board, rather than per part. One panel of ten boards with ten BGA, in total, will cost a little less than ten individual boards with one BGA each.

Stay tuned for my next few posts where I'll cover the pluses and minuses of different panelization techniques.

Duane Benson
"I looked outside my window and what do you think I saw?
The strangest sight I've ever seen you'll never guess just what I mean,
I can't believe it myself"

VLV - Very Large Vias

I recently received a question over on Twitter. Tomaž Šolc, AKA avian2 asked:

"@pcbassembly What is your opinion on the "one big plated drill in QFN ground pad" pattern? pic.twitter.com/M9ZLftpuo0"

From Avian2 Ban_N62IEAAm3acI answered back: "Bad for machine assembly, okay for hand assembly." That's definitely true, but it's worthy of a bit more explanation. Here's the photo avian2 included along with the question. We're looking at the side opposite of where the QFNs are mounted. The two big openings in the square gold pads are the big vias (plated drill).

This is often done when hand soldering QFNs. Somehow you get the little pins on the outside edge of the QFN soldered down. Then you turn the board over and poke your soldering iron into the big opening to solder the pad down.

Generally, there wouldn't be any reason to do this with machine assembly, as we do here in our plant. You put a number of small vias, cap them, and segment the solder paste layer (refer to this post and this post). Thus, we would never recommend using big vias like this for machine assembly.

However, I can envision some situations that might call for this. First, there's the hand solder method I mentioned above. Next, there may be some very specific need to expose a lot of the pad to open air for cooling. In general, this is not the best way to get cooling, but maybe in some special case. Third, perhaps you need access to the pad as a test point and don't have enough room to get access any other way.

You wouldn't do any of those three things in a production environment, but in a prototype world, sometimes things happen differently.

Duane Benson
Hurray! Only one day until Mitten Tree Day!