Category Archives: Screaming Circuits Blog

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

No Open Vias in or Near Pads

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The term “via” probably comes from the Latin word meaning “road.” Therefore, open via would essentially mean “open road.” Open roads here in Oregon sometimes run through open range ranch land. That means that cows have as much right to be on the road as do cars. If you don’t want cows on your PCBs where the BGAs go, don’t leave open vias in the BGA footprint.

Don’t do this:

Here’s what happens:

Your assembled board will likely be missing connection between some of the BGA balls and the board. That’s almost as bad as having a cow step on it.

If you are putting the via in the BGA pad, your only choice is to have the vias filled and plated over at the board fabricator. (Read this post). If you’re putting the vias between the pads, you have two options. You can put solder mask dams on the short trace between the pad and the open via. This will prevent solder paste from migrating. I’ve modified the image below to show what that would look like.

The other, and better, option is to cap the vias with solder mask. This gives a bit of extra protection in case any of the solder mask dams are too thin or chip off. Just make sure you cap these things on the solder side. If you cap them on the back, solder can still spread on the trace and partway into the via. That still puts the electrical and mechanical connection at risk.

Duane Benson
We don’t want cows on PCBs but we have built PCBs that go on cows
The IoC: Internet of Cows

What is a Centroid File?

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Call it what you may, but surface mount assembly robots need a magic file to determine where to place your components and how to orient them. We call it a centroid. What is a centroid file and why is it important to your PCB assembler?

Many assemblers use automated equipment to place surface mount components on PCBs. One of the tools we use to rapidly program these machines is the centroid file (aka insertion, pick-and-place or XY file).

Some CAD packages automatically generate this file, some will not. Sometimes you may simply need to modify the file, and some assemblers can make minor changes to the file or create it for you for a small fee.

Ultimately, the centroid file describes the position and orientation of all surface mount components on the PCB. A centroid file includes: the reference designator, X and Y position, rotation and the side of board (top or bottom). Only SMT parts should be listed in the centroid file the basic format for the centroid file is a comma delimited (.csv) file with data in the following order: RefDes, Layer, LocationX, LocationY, Rotation.

Here’s a breakdown of the data:

RefDes
The reference designator that matches your BOM and PCB markation.

Layer
Either the word “top” or “bottom.” This is not necessarily the CAD layer designator. Just “top” for a part located on the top of the board and “bottom” for parts on the bottom side of the board. Top is often referred to as the component side and bottom the solder side by assemblers and fabricators.

Location
The “LocationX” and “LocationY” values describe the part’s offset from the board origin. The location values require that the part origin be centered in the part. The board XY origin of 0,0 is in the lower left corner of the board. The 0,0 origin for the bottom of the board is in the lower left corner, looking at the top of the board, though the board. Preferred units are in inches (0.0000″).

Rotation
Rotation goes counterclockwise for all parts on top and clockwise for parts on the bottom. In both cases, this is from the perspective of looking at the top of the board. For bottom side parts, it is looking through the board, still from the perspective of looking at the top of the board.

LED & Diode Markation Guidelines for PCBs

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Have you ever had an LED or other diode placed backwards? PCB assemblers work hard to place every component from the largest, highest pin-count logic chip down to the smallest passive components and micro wafer-scale BGAs correctly every single time. A key element of that accuracy is our understanding of your board and the component markings.

If you use surface mount diodes or LEDs, you probably understand the challenges involved in correctly and consistently indicating diode polarity. LEDs are usually cathode negative, while zeners and uni-directional TVS diodes can be cathode positive. Barrier diodes can be either orientation. It all depends on whether the diode is a rectifier, an LED, a uni-directional TVS, part of a daisy-chain and a host of other considerations.

When you start looking at the CAD libraries, you not only have all the differences from that manufacturer, you may also have different markation schemes from each CAD package developer and from each library builder.

Guidelines for diode polarity mark silk-screening — the diode symbol, “K” for cathode or “A” for anode. To ensure the best accuracy, we recommend extra care in marking diodes to remove any ambiguity.

The preferred method is to place the diode schematic symbol in the silkscreen. You may also place a “K” for cathode adjacent to the cathode. “K” is used because “C” could imply that the spot wants a capacitor. An “A” adjacent to the anode on the board works too, though it’s less common. If you are producing a board without silkscreen, put the mark in the copper layer or submit a clear assembly drawing with the other board files.

Relying on +, – or _ are not definitive in what they indicate and are not recommended. For example, a “+” or “-“ sign isn’t good enough, because it’s not always true that current flows through a diode from the anode to the cathode. For the common barrier diode or rectifier, it’s a pretty safe bet. However, with a zener diode or TVS, it’s not necessarily true. That is why marking a diode on your PCB with the plus sign (+) is not good practice.

Ever Use a TI X2SON Packaged Part?

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The name stands for extra small outline no-lead. It’s a newish package from Texas Instruments. In my experience, TI is one of the better companies insofar as testing and documenting manufacturability is concerned. The datasheet for this device is no exception.

The TI part is the five-lead thing above the grain of Jasmine rice, surrounded by a few 01005 ceramic capacitors. I’m selling the capacitors for $500 each. (Just kidding.)

The part is 0.8 x 0.8mm, with the five leads. TI suggests either a 4 mil (0.102mm) trace coming out of the center pad, or a 4 mil via in the pad (the via must be filled and plated at the fabricator ) to escape the center pad. They also do a nice job of detailing out the solder paste stencil layer, as in the following image:

You’ll most likely need a custom CAD footprint for one of these. Either very carefully do it yourself, or go to a solid source like SnapEDA. If they don’t already have it in their library, they’ll make it for you.

These small packages aren’t going away. We’re only going to see more of them. They may seem intimidating, but with a good footprint and a competent manufacturer, they aren’t so bad.

Duane Benson
“A ruler of follows”? That makes no sense.
How about ” a rule of followers”?

http://blog.screamingcircuits.com

Top 5 Ways to Mitigate Component Availability Problems

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Most of the electronics design world is by now aware that we’re in a very serious period of components shortages. Hardest hit seem to be ceramic capacitors, but other passives as well as a variety of connectors and silicon parts are also caught up in the shortage storm. Allocation and shortages hit every few years, but this one seems to be the worst in recent memory. It could be a problem until 2020, and the supply chain and world of components manufacturers will likely be a different animal coming out of it.

So, you might ask, isn’t that just a problem for high volume producers? No, I would answer. It affects anyone regardless of volume. The exact way that it hits you and what you can do about it may differ, but it has or soon will hit all of us.

Here’s five things you can do to minimize the effects. I’m going to go backwards and starting with the most important thing for people who need low volumes manufactured:

1. Check the availability of all of your parts immediately before sending us your bill of materials.

The. very. last. thing. before sending us your BoM. It’s not uncommon for a part to be in stock one day and out the next. We’ve even seen cases where the part’s in stock in the morning and out by the afternoon. If you’re having us quote and order your parts, verify they are in stock as the last thing you do before sending your files to us.

Almost every BoM we see these days has one or more parts that are out of stock. We send you an email about the parts being out of stock. We can’t do anything else until we hear back from you. We can’t build without parts and we don’t know your design like you do, so we can’t guess at substitutions. A last-minute check can save days of delay.

2. Put one or two alternate part numbers in your BoM, especially for passives.

As I said above, we don’t know your project so we can’t pick a sub for you. Give us some alternates. Put them on the same line as the original part, to the right. And be sure to tell us in the special instructions that you’ve put alternates in the BoM.

 

 

 

 

 

 

 

 

3. Consider your parts values carefully. You may be able to pick something with better availability.

The 0.01?F capacitor is the hardest hit component. It’s the most commonly used bypass capacitor. Some designs need exactly that value, but many don’t. It may be easier to find a 0.022?F, a 0.0047?F, or something else close enough. If that’s the case, choose a close enough value that has better supply, or put one in as an alternate.

4. You might need a slight redesign to use a smaller package.

Since smaller packages can be used in more applications, many suppliers will be allocating more of their foundry capacity to smaller form factors like 0402 and 0201 sizes. Some component manufacturers have said they’ll be permanently discontinuing anything bigger than 0402 parts except when absolutely necessary.

Stick with 0402 size passives. It may be easier to find the parts you need in that package, and those size parts will be the first ones to come back in stock.

5. If we send you a message about a part we can’t find, respond as quickly as possible.

We do our best to avoid any delays in this process, but we can only do so much. Help us out by getting back to us as soon as possible, and don’t be afraid to give us more than one part number to try.

This can be a pretty annoying problem and it can cause delays and other problems. The good news is we’re having this problem because the design world is booming and technology is advancing. It will get better, and following these five tips can help prevent delays. Don’t forget to check your parts for availability right before sending your BoM in to us. I mean it!

Duane Benson
Parts, parts everywhere, but not an 0805 to solder

Embedded Passives

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I recently participated in an Altium podcast where I discussed the origins of Screaming Circuits, some DfM hints and a few other topics. I was discussing some of the challenges everyone is having these days in procuring components, and the host asked if we see many people using embedded passives as a way to mitigate supply difficulties.

I told her that I don’t think we’ve seen very many embedded passives come through our shop; too late realizing that given that embedded passives are — embedded — inside — the PCB, we wouldn’t actually see them.

Then, somewhat coincidentally, yesterday I was visiting our San Diego PCB Design division and that very subject came up. It seems that our SDPCB layout group does sometimes use embedded passives in some of the boards they lay out. I need to have a conversation with them about the layout and fab implications of embedding passives.

I’m kind of guessing here at what they look like, so don’t take this representation as literal fact.

Originally embedded passives were invented primarily for space savings. Now with 0201 and 01005 components available, that’s less of a need these days, but embeddeds can still be advantageous for reduction of parasitic effects or in areas where even 01005s are impractical, like termination of large numbers of transmission lines.

What I’m wondering, is if embedded passives could be a viable solution to some of the supply chain issues we’re seeing lately? If you need a few dozen 0.01 uF bypass capacitors on your PCB, but can’t find them*, would embedded capacitors be a practical solution?

*It’s important to note, that in most cases, the term “embedded passives” refers to the process of using various resistive and dielectric materials to create the components within the layers of the PCBs. I’m not talking about embedding the currently hard-to-find discrete resistor and capacitors within the bare board.

Duane Benson
Sorry. I have nothing snarky to say, and as you know, if you don’t have anything snarky to say, you shouldn’t say anything.

What is Your Supply Chain Telling You about Packages?

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Have you purchased any electronics components lately? Have you tried and failed to do so lately? Allocation is the word of the day and substitutions are your friend.

Many, many parts are in short supply, or unavailable with extraordinarily long lead times. Sure, that happens every now and then in this industry. It’s a periodic nuisance, but what should you do for the long term? We’re are getting some interesting stories from component suppliers that might help. 

What we’re hearing is that many passive manufacturers will be trying to move their customers to smaller sizes. They want to consolidate on as few packages as is possible. That means we may be seeing the end of 1206, 0805, and maybe even 0603 form factors for many passive values.

It kind of makes sense. Right now, there might be several dozen different varieties of 0.1uF, 16V capacitor. Does the industry need that? And if there isn’t enough fab capacity to make all of the variations, why not consolidate and run more of fewer variations? It won’t surprise me if we start seeing fewer voltage ranges as well. In most cases, a 16V part will be just fine if you’re calling for a 6V or 10V part.

The chip industry has been doing this for a while. Many of the newer components just come in BGA or QFN packages. Fewer and few leading edge parts come in large through-hole or SOIC packages.

Consider using smaller components, like standardizing on 0402 parts. I know it can be a pain to use smaller parts, but any potential for future proofing your design now can prevent delays or otherwise unnecessary redesign cycles. You might just be able shrink your board size and save some money on the board fab too.

Keep approved substitutions close by, and look for newer chips that are more likely to stay in production. For microcontrollers, pick parts that have multiple memory capacity or speed range variants all in the same package.

This looks to be a pretty extreme allocation cycle, and I have a feeling that the industry will be different when we come out of it.

Duane Benson
Which is worse
Being the missing link or the weakest link?

 

A Question on Schematic Style

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Just a quick question here…

I’m placing the components for my TinyFPGA based stepper motor controller board (see the prior articles here first, here next, and here last). Specifically, I’m finding places for all of the capacitors. That’s where the question of schematic style comes in. 

When I first started using CAD software, sometime shortly before the stegosaurus roamed the land, I would position component symbols on the schematic near the chips they belonged with. Doing so made it easy, come layout time, to remember which capacitors go where.

Since then, though — and I don’t remember when I started this — I’ve started following the practice of grouping capacitors on the schematic, as I’ve done on this sheet to the right. All of the capacitors are up at the top, shown connected to V+ and ground rails.

That’s not a problem when they’re all 0.1uf bypass capacitors and each chip just takes one. However, with higher speed and more complex chips, multiple bypass caps or different values are often required on each chip.

Now, when I go to layout, I need to go find my component data sheets again (they really should never be very far away) and re-figure out what combinations of bypass capacitors go to which pins on what chips.

I like the cleaner schematic that results from grouping bypass caps, but it’s adds pain and a bit of opportunity for error during layout.

What style do you use and why? Am I an idiot for doing it this way? Wait. Don’t answer that last question.

Duane Benson
Six of one and 12 × 5 × 10-1 of another

5 Traps to Avoid When Changing from Hand to Machine Assembly

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In the past, it was usually pretty easy to find chips in both surface mount and through-hole packages. Somewhere in the past decade or so, component manufacturers stopped introducing through-hole versions of their newest chips as standard practice. In many cases, new components can only be found in tiny QFN (quad flatpack, no leads), or wafer-scale BGA (ball grid array) packages.

The maker community, never shying away from a good hack, found ways to work with many of these parts while still hand building. There are very few components used in the pro-design world that are still unusable by a creative DIY maker.

But what happens when a maker has a great design and wants to mass-produce it?

Sometimes the techniques that make things work when hand soldering, will completely break a machine assembly process. To cure that ailment, I’ve compiled five common traps to avoid when moving from hand to robotic assembly.


5. Consider moisture sensitivity. It may not seem logical, but plastic does absorb moisture. And, it doesn’t have to be dropped in the sink for it to happen. Just sitting around exposed to the air, plastic chips will absorb humidity. In a reflow oven, these parts can end up acting a bit like popcorn. The moisture turns to steam, and if it can’t outgas fast enough, may split the chips open. Often, the damage isn’t visible to the naked eye, but with show up as an unreliable product in the field.

When we DIY folks hand-build boards, we tend to open the component packages and then just let the parts lie around without giving thought to proper storage. If you are going to send your project off to be machine assembled, you can do two things with moisture sensitive parts.

First, order the parts when needed, not before, and keep the packages sealed. Alternately, you can send in parts that have been exposed to the air; if you inform your assembly house that the parts are moisture sensitive, and ask that they be baked prior to assembly. Prebaking will remove the moisture safely.

4. Don’t skimp on solder mask. Some board fabricators offer reduced prices if you order your boards without soldermask or silkscreen. That’s not a problem when you’re hand building — you can regulate the amount of solder by eyeball.

However, when a stencil is used to apply solder paste and the board is run through a reflow oven, the solder will spread back on the exposed copper traces. This may leave your parts without enough solder on the pins to create a reliable connection.

Solder mask may add a bit of cost up front, but will increase reliability and reduce cost in the long run. Creative choice of solder mask color can also add some personality to your boards.

3. Silkscreen is important too. Lack of sIlkscreen isn’t a reliability issue, but it can make accuracy of assembly more difficult to achieve. In a perfect word, the CAD files would tell the assembly machines exactly where each part is supposed to go and what angle and orientation it needs.

Unfortunately, we don’t live in a perfect world (who knew?). It’s far too common to have footprints with errors in them, or components with ambiguous marking, to depend on the CAD files alone. Clear silkscreen will help to ensure that any errors in the data are caught visually.

If you don’t want to clutter your PCB with reference designators and polarity markings, put the designators and any other important markings in the document layer in your layout software. Then, tell your assembly house to look on that layer for the information.

2. No need to fear surface mount. One of the easiest ways to ensure that a board can be hand-built is to stick with through-hole parts. But doing so puts many limits on a design, and rules out a lot of new technologies.

Little breakout boards — a small surface mount chip pre-mounted on a PCB, with hand-solderable headers — are available for a lot of new parts, but not all. That’s helpful, but they take up a lot of extra board real estate and cost more that the part alone.

If you’re hand building a prototype, or a small number of boards for your own use, go ahead use a breakout board. But, when it’s time to get a thousand built up to sell, re-layout your PC board to use the chip without the breakout board. Just don’t forget the bypass capacitors or any other required support components.

As a bonus, many breakout boards are open source, so you may be able study and use a proven schematic and layout for that part of your design.

1. No open vias in pads. QFNs and BGAs have pins/pads under the part, often completely inaccessible. That’s fine for a reflow oven, but what if you’re soldering it by hand?

A common hand-soldering practice is to put large vias in the pad. Fix the part onto the board with tape. Then, turn the board over and stick solder and a small tipped soldering iron through the via. By doing this, you can hand solder almost any leadless surface mount part.

You can probably guess that I’m going to tell you open vias in pads will not work with automated assembly. The solder will flow down the via and end up on the back side of the board. You may end up with shorts on the back side, and parts that fall off of the front side, or just don’t connect with all their pads.

If you use the open via hand solder technique, you’ll need to re-layout your board without any open vias in the pads before sending it for manufacture.

0. Go for it! It wasn’t that many years ago when the tools and services necessary to get an electronic product manufactured were so complex and expensive as to pretty much make it impossible for DIYers to turn a hobby project into a small business. Times have changed, and with those changes, the hardware startup is back — and within just about anyone’s reach.

Duane Benson
Breaker, breaker, one nine, clear the line, we’ve got boards to build

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