Start the Year Right, Without PCB Placement Overlap

Today’s illustration isn’t a super-bad problem. You can usually make this work — unless  you’ve got to align with a hole in case. I’m talking about the venerable 3.5mm audio jack. They aren’t used all that often these days, but when they are, one of the most common formats has a design detail that makes edge alignment pretty critical.

The part of the connector that receives the jack is a short barrel, with an outside diameter larger than the height of the rest of the connector, as you can see in the image on the right. It comes in thru-hole and surface mount varieties.

This means that you have to have your solder pads or holes just the right distance from the board edge. Too close, and you can violate design rules. Too far inset, and you won’t be able to mount the connector flush.

 

 

 

 

 

This part can cause additional problems if the board is panelized. Like other overhanging connectors, the panel tabs, panel rails or other boards in the panel may make it impossible to mount the part, even if the spacing is correct.

The board shown below has both incorrect spacing, and another board in the panel blocking placement. The surface mount pads allow for more flexibility in positioning — it would have worked if not in a panel.

 

 

 

 

 

I’ve done this myself. Speaking from experience, I can say that it’s easy to avoid, and quite sad when discovered at assembly.

Duane Benson
Down at the edge, close by a panel rail
Close to the edge, round by the routing tab

Top 10 PCB Assembly Tips for 2016

I’ve already written my top 10 predictions for the coming decade, in this blog post. But, while predictions might be fun to muse upon, they really won’t help you get your job done. My top-ten8 pieces of PCB assembly advice for the coming year should make up for that.

000

Before you even start component selection, give thought to the design scale. What’s more important, board size, cost, or time to layout? A large board will be easier to route, but will cost more for the fab. A smaller board will cost less for the fab in terms of square inches, but may cost more due to extra layers, and may take longer to layout.

001

Factor in the cost of component size. For passives, roughly 0603 size parts will probably be the sweet spot in terms of lowest cost. The 0603 is also a good size for overall handling. We’ll assembly down to 0201 parts, but not all manufacturers will. 0603s are also easy to rework, and are manageable if you feel the need to hand solder a few.

010

Check out any exotic or very new parts. Some parts, these days, are only available in super small wafer scale BGA, or small QFN form factors. Take a look at your integrated circuits and make sure they come in packages that you’re comfortable working with.

011

Check for sole-source parts, or low-availability parts. The last thing you want is a completed design that’s sitting around waiting for one long-lead time, sole sourced part. If a sole-sourced part is at risk for availability, you might want to find something similar and more available.

100

Don’t forget manufacturing thermal concerns when laying out your board. Very large parts next to very small parts can cause problems. The large parts will act a bit like a heat sink and may prevent the solder for the small part from melting properly. The same thing can happen with internal copper planes that overlap on half of a small part, but not the other.

101

Give extra care to the clarity of reference designators and polarity markings. Make sure that it’s very clear which designator goes with which part, and that there isn’t any ambiguity in polarity markings. Take special care with LEDs, as manufacturers sometimes swap polarity markings between the anode and cathode – yes, the exact same mark can mean anode on one LED and cathode on another. Also, do your best to keep reference designators off of vias or any other spots that might break up the text.

110

When you’re ready to send your project our to be built, give your files a double check to makes sure you have the correct versions. bills of materials are especially susceptible to having bits of information out of date that might cause delays.

111

If you’re sending in a parts kit, double check that you have all of the parts, and that you have part number and reference designator on the individual part bags.

Manufacturing is just putting parts on boards, but it’s doing so with a whole lot of variables. A few extra checklist steps can go a long way toward removing variability of those variables.

Duane Benson
I am one with the net force. The net force is with me

Suspect Through-Hole Packaging

Surface mount components are carefully packaged up in strips, tubes or trays, because they’re machine-assembled. The assembly robots need order and organization to properly do their job.

Through-hole parts, on the other hand, are almost always manually inserted by actual human-type people. That being the case, the manufacturers and distributors are sometimes more lax with their packaging. They assume that, since a human is picking the parts, a jumble is okay. Sometimes it is, but not always.

In the case of these through-hole DB-25 connectors, the jumble was too much and lead to a number of bent pins. Slightly bent pins usually aren’t a problem, but some of these ended up a lot more than “slightly” bent.

To make matters worse, these pins are small thin-wall tubes, which are more susceptible to breakage when bent than are solid wire pins. For the connector on the bottom of the image, some of the most horribly bent pins may not be straightened without breaking. If they are, they’ll certainly be weakened.

The moral of the story is that through-hole parts need care too. We can’t toss them around just because they aren’t the latest technology.

Duane Benson
Spider-Pin, Spider-Pin,
Does whatever a Spider-Pin does

7 Cost Reduction Design Tips For Makers

As a maker, you really need a decent price, with good quality and good service. Contrary to what many people think, you don’t need to look outside of North America for this. 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, let’s take a look at seven design decisions that can make your manufacturing more affordable.

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

Avoid leadless packages Like QFNs and BGAs. Screaming Circuits builds 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 and continuous strips. To save costs, use full or partial reels or continuous strips of at least 12″ long.

Stick with surface mount. These days, through-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 through-hole, too. If you do need a few through-hole parts, this is an opportunity to put in a little sweat equity by soldering the through-hole yourself and save a bit of money.

Keep surface mount parts on one side. Putting surface mount parts on both sides of the PCB is a great way to better utilize space. However, if cost is more of a concern, and you only have a few parts to put on the back side, it may be more cost effective to move them to the top side. If you’ve got a lot of parts, the additional cost for assembling both sides may be less than the cost for the extra board size, but with a small number of parts that’s probably not the case. Quote it both ways and see which is less expensive.

Panelize small boards. Sticking with a larger size makes the job easier, and, again, creates extra savings. If your board is smaller than 16 sq. in., panelize it.

Save on start-up costs. Just the act of starting out can pretty much break the bank. Software like PCB123 offers full-featured PCB CAD systems you can get free of cost.

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

Duane Benson 

 

Fab Drawings or Assembly Drawing Standards?

It’s not always possible to have all of the information needed for a successful PCB assembly printed on the blank PC board. When this is the case, we ask for an assembly drawing – like I suggest here. But what about things that are important at the PCB fabrication stage rather than at assembly.

That’s where the fab drawing comes in.

One of the problems with this system is that the “standards” for fab and assembly drawings are only loosely adhered to, if you can call them standards at all. If in doubt, label the fab drawing “Fab drawing” and the assembly drawing, “Assembly drawing.” That may seem obvious, but in the wide world of technology, obvious too often is anything but.

benson-oct25

(image from xkcd.com)

Clearly label anything that the fab shop needs that isn’t obvious from the Gerber files, make a PDF, and label it “Fab drawing.pdf.” Do the same for any assembly information and instructions and label it “Assembly drawing.pdf.” If information is needed by both the fab shop and us, the assembler, put it in both drawings.

We recently had a case where a component polarity wasn’t marked on the board or in the assembly drawing, but was in the fab drawing. We do our best to catch such things, but it ads a bit of ambiguity to the process. If you’ve been reading this blog before, you’ve likely picked up that I do not like ambiguity. I do not like it one bit.

Duane Benson
Vote for clarity! Kick ambiguity out to the street

http://blog.screamingcircuits.com

Accursed Diode Marking

Am I a broken record? Pretty much — especially when it comes to confusing diode marking.

For example, everyone knows what the diode symbol looks like, and pretty much everyone knows which side is the anode and which is the cathode. Right? It’s just like in the following picture:

10 designersnb figure 2

Is that big enough?

Normally, the clearest way to indicate polarity on an LED is to put something like this diode symbol in silk screen next to, or between, the copper pads. In theory, that should remove ambiguity.

Ambiguity in marking is the enemy of polarized parts. Unfortunately, as I cover in this, and many other blog articles, LED manufacturers seem to conspire against us all when marking is concerned.

We recently ran across a case of built-in ambiguity. The PCB had, what looked like, a very clear marking. The image on the right is from the assembly drawing, which is just a blow-up of the board silk screen and documentation layer.

With that marking, I’d quickly come to the conclusion that the anode is on the right and the cathode is on the left. I’d even confidently state that it’s a sure thing and extremely unlikely to cause any problems. But …

Here’s where I’d be very wrong, and why it’s so important to always check the datasheet when dealing with diodes. Take a look at the following clip from the component’s datasheet. Scroll down to the bottom of the image for the punch line.
10 designersnb figure 3

Wow. I can’t even …,

The board designer was just following the datasheet. That’s a perfectly proper thing to do, except when the manufacturer flips a coin, as it appears to have happened here. In this case, dispense with the symbol altogether and use “A” for anode and/or “K” for cathode in the silk screen. (Use “K” because “C” looks too much like a reference designator for a capacitor.)

Duane Benson
In the land of the insane, only the sane are crazy

http://blog.screamingcircuits.com/

What Route Do You Take?

There are a lot of polar opposites in the “what is my philosophy” world: Mac vs. PC, on shore vs. off shore manufacturing, Ford vs. Chevy, Atmel vs. Microchip (well, maybe not that one so much any more), auto router vs. hand route…. Yes, I’m specifically avoiding political opposites.

DB 1Routing is what I’m really interested in today. The conventional debate is hand vs. auto route. CAD companies spend a lot of time and money on autorouters, but there’s definitely a line of thought that says it’s not ready for prime time yet. This shirt designed by Chris Gammel, on Teespring pretty much says it all.

But, it’s more complex than that. Most auto-routes end up requiring some hand work, either to finish routes that can’t be found automatically, or to clean up a few less than efficient choices. There are differing techniques for complete hand-routing as well.

I often find myself looking at a layout project a bit like a chess game. I don’t just start at one end of the board and work my way to the other side. I tend to focus on specific parts or critical requirements first, like signal paths that need to be short, or sections with more critical grounding requirements. (The image above isn’t mine. It’s from the Beagleboard.)

When it gets to the mass, I tend to try and think ahead, projecting moves out, as though it were a chess game. When I’m looking for the best route for signal path A, I try and think ahead to how it will impact B, D, D… as far ahead as I can go.

I’m not sure if doing it this way is easier, of if it would be better to just start routing and then re-route as I run into roadblacks. What about you? How do you approach a complex layout?

Duane Benson
Holy cow. I Googled “Trust no one” to get some ideas for my signature
Never do that. It’s going to take a week to shake off all the negativity

blog.screamingcircuits.com

Building Boards for the Intel Edison

I’ve recently spent some time getting familiar with the Intel Edison. The Edison has a dual-core 500MHZ Intel Atom processor, with built-in Wi-Fi and Bluetooth. It comes with 1GB of RAM, 4GB of eMMC internal storage, and a USB 2.0 OTG controller. It doesn’t bring any of the connectors (power or signal) out in a usable form. Rather, it’s designed to be plugged onto another board through a 70-pin high density connector from Hirose.

I designed a small board with I2C (both 5V and 3V connectors) and a micro-SD card slot. My board still doesn’t have the power or console connectors. For that, I’m using a base board from Sparkfun.

Figure 1

Figure 1

Step one of the assembly process, is, of course, to design and layout the board. Using the Sparkfun open source designs as a jumping off point, I ended up with the nice, compact layout (1.2″ x 1.75″) shown below in Figure 2.

Figure 2

Figure 2

After getting the files ready and placing a turnkey order on our website, I followed the board through with my camera. Here it is after offline setup, with the parts ready for robot pick-and-place:

Figure 3

Figure 3

In one of our Mydata My500 solder paste printers:

Figure 4

Figure 4

On the pick-and-place machine, with solder paste, but before any components are placed:

Figure 5

Figure 5

The parts plate in the machine:

Figure 6

Figure 6

 

With most of the components placed:

Figure 7

Figure 7

Through the reflow oven, prior to final inspection:

Figure 8

Figure 8

The final product, top view:

Figure 9

Figure 9

I abbreviated the process a bit, but those are the major process steps along the way.

Duane Benson
Happy birthday (month) Nikola Tesla

Where to Put Panel Tabs

Many small quantity PCBs are ordered individually cut. They come to us as a set of unconnected boards. For small quantities of reasonable size boards, it makes the most sense to order them this way. However, for really small boards, and larger quantities (50 or more), purchasing boards in a panel (also called an array) is more appropriate. It reduces errors and assembly time.

There are a few additional factors to consider with panelized boards.

  • First, don’t try to create a panel in your CAD software. Just lay it out as a single board and have the fabricator put it in a panel. You’ll get the most efficient use of PCB space that way, and the fabricator will create the files in the format that the assembly shop (Screaming Circuits) needs.
  • Avoid family panels. A family panel is when several different boards are put onto the same panel. The boards in family panels often repeat reference designators, which causes problems at assembly. See this blog article on how to properly assign reference designators on a family panel.
  • If you have overhanging parts, like the increasingly common micro USB connector, make sure that the panel tabs aren’t placed near the overhanging them.

This blog article gives some background on the connectors.

Some components, such as the connector in the link above, have protrusions that will keep them from laying flat on a panel tab. In all cases, even without the protrusions, the operation of separating the panels with a component on the tab can weaken the component solder joints, or even pop it off the board completely.

How not to do it:

Figure 1

Figure 1

Instead, make sure that the tabs don’t end up under your overhanging component. Have the tab moved like this:

Figure 2

Figure 2

You can put this instruction in the document layer of your CAD file, or in a separate document covering fab instructions. In the CAD image below, the overhanging component has a keepout area. The document layer has instructions to keep panel tabs out of the area.

Figure 3

Figure 3

If in doubt, don’t hesitate to contact us or Sunstone Circuits directly to ensure that your instructions are clearly communicated.

Duane Benson
Wood paneling as a wall covering is really out of style

http://blog.screamingcircuits.com/

Top 5 Things to Know When Moving from Hand Assembly to Robotic Assembly

A lot of factors go into the decision to hand build or outsource circuit boards. I hand build my own sometimes, simply because I enjoy the challenge. Of course most of the projects I design are for my own use, so timeliness isn’t that important. When I do design something that will go out to a customer, like my electronic business card holder, I will send the board through our shop. In those cases, quality is important, as is delivery, and the quantity is often too high to hand build. Machine building also allows me to use smaller and more complex parts. sc1

That same decision — hand build or outsource — takes place in the heads of designers all over the country. When the decision is to outsource, there are a few important things to consider. Some things that work fine when hand soldering may stand in the way of quality, repeatability, and reliability when machine assembling.

Here are five of the most important considerations when changing from hand-built to outsourced at a place like Screaming Circuits

1. Use solder mask and silk screen. A good solder joint needs the right amount of solder in the right place. Solder will tend to flow down bare copper, bleeding outside of the area it belongs, and down exposed copper traces and vias.

The main purpose of solder mask is to keep the solder where it belongs. It also protects the traces, but that’s a longevity issue. Solder bleeding is a manufacturing and reliability issue. This isn’t a problem when hand soldering. In fact, it can even work to your advantage when hand-soldering really small parts. It gives you more room for your soldering iron to hit metal.

Not so with solder paste and machine assembly. Use solder mask.

2. Avoid the pseudo panel. Keeping small boards in a panel is the recommended best practice in the manufacturing industry. We appreciate it and, while not always necessary, it can reduce your costs. We sometimes see what we call a “pseudo panel.” This is a board where multiples of the board are put in the same PCB, like a panel, but unlike a panel, the boards don’t have routing or V-score between them. Sometimes the designer will put a bunch of vias to outline the board, or just ask that we use a band saw to separate them.

That’s a time-consuming, expensive and potentially damaging process. The vibration of the saw can crack solder joints, and, you’re unlikely to get boards that are all the same size. Have small boards panelized by your board house.

3. Family panel (pseudo or not). Similar to the pseudo panel is the family panel. A family panel is a case where a project is made up of several different PCBs, and they are all laid out together, as though they are one design. If the board isn’t routed between the designs, you’ll have the pseudo panel problem described above.

The bigger problem, though, comes with reference designators. We typically see family panels with duplicate reference designators. Each design, for example, will have its own C1, R1, Q1, etc. We use the reference designators as position identifiers: If you have three different parts labeled R5, our machine programmers will have a problem with it. It’s even worse if the values differ; on one design, C1 is a 0.1uf capacitor, while on another design, it’s a 22pf cap.

If you’re making a family panel, give each and every placement a different reference designator. One way would be to us extra digits. For example on one design on the family panel could have C100, C101, C102… The next would be C200, C201, C203, and so on.

And don’t forget the routing or V-score between the designs.

4. QFN — hole  in the middle. A common technique in the hand soldering world, for QFNs and other parts with thermal pads underneath, is to put a big via in the middle of the center pad. By doing so, you can stick a soldering iron and some solder down through the hole and get a good solder connection on the bottom pad.

This doesn’t work with machine assembly. the solder paste will flow down and out the hole in the reflow oven. You’ll end up with a poor connection (or no connection) to the thermal pad, and solder slop on the back side of the board.

5. Parts and the bill of materials (BoM). When I build my hobby projects, I often get a bit carefree with the bill of materials. It’s not good practice, but I do. I’ll put a part in the BoM that I used before, and not check to see it’s still in stock. I’ll put parts in the BoM with just the values and not any part numbers. Things of that sort require tribal sc2knowledge, which only the designer has.

When building, sometimes I’ll just grab a part that’s close. If I need an 0805 1uf, 10V capacitor, I can grab a 16V, 25CV, etc. I can even make an 0603 part work. You as the designer may know that something close will work, but an outside house can’t know. You need to tell them exactly what the part is.

Before sending anything through our shop, I do clean up the BoM. In order for us, or any manufacturer, to build the boards, the BoM needs:

  • A unique reference designator for each part placement.
  • The quantity of each part used on the board.
  • The manufacturer.
  • The manufacturer’s part number.
  • DigiKey part numbers can be used as well.

Here’s our website page explaining the BoM format in more detail.

The transition from hand building to outsourced machine building can be an intimidating one. But, with a few considerations, it can be an easy and rewarding transition.

Duane Benson
Put the right part in
Put the wrong part out
Put the right part in
But please don’t shake it all about

blog.screamingcircuits.com