Never Take Pin Numbering for Granted

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

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?

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?

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 – For Big Parts Only

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

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

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 Do You Do If You Can’t Have Reference Designators?

The first answer to that question is probably going to be along the lines of, “Put them on the board.”

But, sometimes you can’t have reference designators on your board. Maybe it’s too densely populated and there isn’t room. Maybe, for aesthetic reasons, you’ve chosen to leave them off. With some products, like development boards, it’s sometimes necessary to use the space for instruction or functional identification and reference designators would confuse your customers. Figure 1

It’s always best to put reference designators as close to the part as possible, and on the same side as the part, but if that’s not possible, you can still create an assembly drawing. When laying out the board, put the reference designators in a different layer than the text you want in silk screen. Then, create a PDF that has all the component outlines in their place, with reference designators. Make one for the top and one for the bottom. Call this document “assembly drawing” and include it in the files sent in to be manufactured.

Figure 1 shows a good assembly drawing format. It has reference designators and polarity marks.

You might ask why reference designators are needed when all the surface-mount parts are machine-assembled. First, any through-hole parts are hand-assembled. Their locations and board side needs to be clear for the people stuffing them.

Second, CAD systems don’t always have 100% accurate information. If the center point of the footprint is off, surface mount machines (ours and anyone else’s) will center the part where file says to put it, which, in the case, would be the wrong spot.

The reference designators are also part of quality control. It’s another opportunity to remove ambiguity. Ambiguity bad. Certainty good.

Duane Benson
Car 54, where are you?

http://blog.screamingcircuits.com/