Blog - Screaming Circuits


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"

Component Packages - Let's Get Small

I've been on a bit of a package binge lately. First talking about metric vs. US passive sizes, and then a very tiny ARM Cortex M0 from Freescale.

The Freescale BGA part checks in at 1.6mm x 2mm. That's cool and I'm almost always in favor of making things as small as possible, but, as I wrote in my prior blog on the subject, it's not always possible. The 0.4mm pitch BGA is problematic unless you can spend a lot of money on the raw PC boards, or will have super high volume.

Small boardAll is not lost, though. You still can use a tiny ARM Cortex M0 part. Just not quite as tiny. That same part also comes in a 3mm x 3mm QFN package. You lose four pins (16 vs. 20) going from the BGA to the QFN, but if you can handle that, it's a very viable option that doesn't require any exotic PC board technologies.

A few years ago, QFN's were scary, but not so much any more. I've designed a few of them in using Eagle CAD. Just be sure to pay attention to the footprint. A 6 mil trace is more than small enough for a 0.5mm pitch QFN.

Duane Benson
Strive at all times to bend, fold, spindle and mutilate

Beginning FPGAs

An hour later, you can join me to explore the very basics of Field Programmable Gate Arrays in FPGAs - Light at the End of the Learning Curve.

The session is on Tuesday, April 1st, from 3:30 - 4:30, in SJCC 210C

FPGA talk EELive 2014

Best Practices in PCB design

If you do happen to end up at the EE Live show, be sure to catch my session: Best Practices in Circuit Board Design. I'll show the most common traps we see that can cause manufacturability issues.

The session is on Tuesday, April 1st, from 2:15 - 3:15, in SJCC 210D.

Best practices talk EELive 2014

Screaming Circuits at EE Live

If you're in San Jose, CA this week, stop by and see us at the EELive show. We'll be in booth 927 - just a short walk, and a left turn from the entrance.

Show hours are:

Tuesday, April 1,    10:30 - 5:00
Wednesday            10:30 - 5:00
Thursday                10:30 - 1:30

Map to booth 2014

0.4mm Pitch BGA is Awesome

I recently had a conversation with a friend about 0.4mm pitch BGAs. The specific part is the Freescale FreescaleKL03KL03 ARM Coretex-M0+ microcontroller in a 1.6mm x 2mm, 04.mm pitch package. That's a 20 ball wafer scale BGA form factor.

I don't have an actual part to photograph next to a grain of sand, but trust me (or don't), it's really small.

Ti 0.44 pitch dimensionsThe challenge, and the reason I suggested a QFN form factor instead, is the costs
involved. If you have the extra budget money for more expensive PC boards, then go ahead and use this form factor. You probably won't be able to use this package in cost constrained situations.

The simple reason is that you can't escape route the inner six pins without using super small vias between pads, or in pads and filled and plated over. The page on the left is from a Ti doc, but any variations in geometry will be minor.

You can see that you can't put a trace between the pads. Maybe a 2 mil trace, but maybe not. There just isn't much room. The recommended method is to put micro vias in the pads and have them filled and plated over at the board fab house. Never put a via in a micro BGA pad unless it's filled, plated over, and flat.

Duane Benson
There are more things in heaven and earth, Horatio, 
Than are dreamt of in your philosophy.
But open vias in pads aren't one of them

When is an 0201 Not an 0201?

Metric vs US resistor packageI'm working on a special project here that involves some 0402 LEDs and 0201 resistors. When doing such a thing, you should always check the footprint you're using against the data sheet. When using extra small parts, like this, I recommend making a custom footprint unless the one you picked is exact, and I mean exact. There just isn't an margin for error at these geometries.

Take a look at the table on the right. The dimensions are in mm. Spot anything a bit off? Counter to most data sheets, the sizes listed in the "Type" column are metric sizes. At DigiKey, the package was listed as "0201 (0603 Metric)." I see that all the time, but for some reason, most data sheets Metric vs US resistor package Conversionshow the package name in US size while listing the dimensions in metric.

The first table was at the front of this data sheet (page 5). The second table was on page 35 - the opposite end of the data sheet.

We do occasionally get boards with metric size pads for a US size part, or vice verse. Sometimes we can make it fit, but not always. Bottom line, is to check and double check. I caught this one because the dimension .54 mm is about 21 mils, which is too small for an 0402. That, and the fact that the table doesn't list an 0201 size.

Duane Benson
Is it Bigfoot or Sasquatch?

 

EELive The Show is On it's Way

If you're going to be in San Jose during the first week in April,stop by the EELive show (formerly known as the Embedded Systems Conference). We'll be in booth 972. Stop by and see what we can do for you. Maybe earn a fabulous T-Shirt.

While you're there, check out my two sessions:

"FPGAs - Light at the End of the Learning Curve", at 3:30 on Tuesday. It's not exactly PCB Assembly, but FPGAs are quite cool and getting more useful by the minute. Get some basic knowledge and see how to avoid many beginner traps.

My second session: "Best Practices in Circuit Board Design", part of the "Hardware Startup Engineering Summit," is set for 2:15 on Tuesday. You can watch that one and then run with me to my FPGA session at 3:30. Place bets on whether I can make it in time.

Duane Benson
Ach, Captain. She Canna Go Any Faster