Flyback Diodes - a Question

A while back, I posted about putting caps in parallel. Sometimes it makes sense to do that either to reduce the effective series resistance (ESR) or to better respond to different frequencies of ripple, spikes or distortion.

But here is my question - Take a look at the schematic below. This is mostly for you motor control and power component folks, but anyone can take a stab at it.

MOSFETs typically have their own flyback diode built in. But it may not be fast enough or good enough in some way or other, so it's common practice to use external flyback diodes in parallel with the internal one. In this schematic, each leg of the H-bridge has three MOSFETs in parallel. It also has D7 and D8 as flyback diodes for the bottom legs. For the moment, ignore the fact that the top legs don't have any external flyback.

Here's the question: Is is equally effective to have a single big flyback for the three parallel transistors as it is to have an individual flyback for each MOSFET?

Duane Benson
Is snow cold? Perhaps, but it's all relative.

All things are relative
All my relatives are things
My relatives took all my things

Silk Screen Marking

When we select our board house, most of us carefully consider space & trace, cost, turn-times and things like that. It's important to not forget about their capabilities regarding the silk screen layer. A great board can easily be made mediocre by poor quality silk screen. A Mediocre board can be made much worse. Ambiguous marking can make either difficult to use.

This board here has silk screen printed right on the pads. Mis-registration like this reduces the solder surface area and can prevent a good connection. See a little more detail in this post. The board house Sunstone.com has a service add-on to clip the silk screen off of the pads which would eliminate this problem.

Then, the ambiguity factor can be a problem with assembly too. Sometimes when parts are very close and the board house can't make fine line silk screening, we can't tell which reference designator goes with which part. And on polarized parts, especially diodes, the polarity markings can be ambiguous as described in this post. Diodes, especially barrier diodes can be a real problem with markation. A simple (+) or (-) just isn't clear enough.

Duane Benson
weiße Zeug auf Silber Zeug ist schlecht

Icky Via Near Pad

Here's a case where the board house didn't do an engineer any favors. There are issues with soldermask opening size, registration and masking between the via and the pad.

The pads are non-soldermask defined as we like them, however, the mask openings are much larger than we would like or than most manufacturers would recommend. Check the component datasheet for the actual number you need to use.

Then, the registration is way off. That alone wouldn't make the board unusable, but it does speak to the suitability of the fab process to this level of technology. Either it was a bad batch or this fab house shouldn't be making boards for BGAs or other fine pitch components.

The worst part is the direct solder path from many of the pads to their vias. It's quite likely that in reflow, the capillary action from the via will suck the solder paste and the solder ball off of the BGA leaving a complete open. Check out this post for a little more information.

Make sure your board house is capable of building to the technology you need. It's not worth the discount you might recieve. And, always give your boards a good look over before sending them out for assembly.

Duane Benson
In this case, BGA = Bad Green Area

Point Of Load Power Supply

POL or Point-of-load seems to be a hot topic these days. Just to be clear, I'm not talking about Internet traffic or construction - although point of load, as well as load bearing capacity are also quite important in those areas as well. I'm referring to power distribution and regulation in electronics systems.

It wasn't that long ago when most things could be run off of +5 volts or +/- 12 volts. That was easy. You just have three power busses and a ground bus. You might have to double some up or take care what is next to what to make sure you have enough current carrying capacity and to reduce noise, but it was fairly easy.

Who does the PCB Layout?

We've done some PCB layout surveys with our customers recently; how long it takes, what is the most tedious task, where do mistakes happen, etc.  One thing we've noticed is that you are tasked with the design and layout for most projects; and sometimes you choose to outsource the PCB layout.

We’ve been doing blog posts that help schematic designers with layout; starting with a five part series titled “5 Basic Steps to a Successful PCB Layout,” with posts on "BGA Layout" and "QFN CAD Libraries".

What makes you choose to outsource a layout?  What have been some of the biggest pitfalls and successes in bringing in an outside resource?  What have you learned from all this?  What are the costs you didn’t account for?

We are asking you to email your stories and experiences to us, to: info @ screamingcircuits.com  and are taking stories for this up to April 30, 2008.

Submitted stories will be compiled for articles in May, if you have something you’ve learned the hard way with outsourcing PCB Layout please share it with us so we can post it for others.

Thank you,

Jered

Product Development Manager

Screaming Circuits

Modules and Daughter Boards.

We ran across an interesting part the other day. Modular sub-components are getting more and more  common. You see bluetooth radios in module form, ZigBee radios, GPSs, Power Supplies and other common building blocks showing up like this more and more.

These little modules are are non-isolated PTH05060WAS and PTH05050WAZ DC to DC converters; less than $20.00 each from Digi-Key as of this writing. One of the interesting aspects of this product is that they are multi-sourcable. In my experience, most modules of this sort are dramatically different from one manufacturer to another. These ones, though, are offered by at least three manufacturers as part of the "Point-of-Load Alliance" (POLA) with a standard footprint and functionality.

It's also interesting to note that these are essentially BGA packages. It's not quite the same as a standard BGA but the connections use solder balls like a BGA.

Soldermask, Tombstoning and Solder balls

This is an update to a prior post about solder mask and tombstoning. In that post I wrote about how having your solder mask too thick up in the pad or poorly registered can cause tombstoning with 0201 and 0402 parts. It can even cause the problem with larger parts like 0603s.

Further, I've also been told that thick soldermask like this can cause solder balls. The solder can get trapped by the part and a small opening in deep solder mask. It has no place else to go, so it can spit out and land someplace near as a solder ball. Not a good thing.

Duane Benson
He shoots. He scores. He shorts!

Ghetto Panelization

Recently, we processed a job that was what I refer to as "ghetto panelized."  Basically, rather than having the PCB fab shop create a panelization for you, ghetto panelization refers to creating your own large panel and separating all the different boards with lines of holes (see image).

People generally ghetto panelize a board solely to save money.  Considering the way most board houses calculate pricing, it is possible to save a bit of money this way.  Take a sample board that is:

• 3" x 4"
• 2 layer
• Tin Lead Finish; non-RoHS
• Electrical Tested
• 4 day processing time
• 100 boards needed

Doing this the traditional way, single up at Sunstone through their Quickturn Boards quote engine, you would pay $15.90 a board. 

Taking this same board, ghetto panelizing yourself to a 3 x 3 panel (Total dimensions of your single board are now 9 x 12), the price for 15 panels (135 boards) drops to the price to $4.61 per unit.  That is a pretty big difference in price.  Overall, you would save $967. It's safe to say that someone would do this for price reasons, not convenience; unless of course you find using a band saw to cut apart PCBs more convenient.

So, why wouldn't everyone want to ghetto panelize boards?  First, it is near impossible to get a precise cut, so each board is has a unique shape (and rough edges).  This will negate any machine assembly house from being able to assemble them post breakout.  You would therefor need to do hand assembly, or have them assembled before you break them apart.

Secondly, reliability is a concern.  Depending on your method, you can damage the board functionally.  This is especially applicable to boards that have been assembled, where certain solder joints can crack.  You are introducing a lot of vibration and force to the board.  A joint that is already weak can snap.

However, if you need a few simple connector boards, ghetto panelizing your PCBs can be a cost effective way to get a few extra boards for a better price.

- Jered

Surface Mount EMI Gaskets

Here's an interesting board.

EMI gaskets have been around for a long time. Some are foam. Some are springy metal and some are wire braid. They all do the same things though - create a seal between your EMI source and your EMI shielding. We ran this board recently that uses a foam gasket from Gore Industries. It's a pretty cool product in that it can be run through our SMT machines just like any other surface mount part. No secondary or hand operations are required.

While I have the picture of the board up, there are two things I'd like to point out. (Always have to be the critic. Right?)

[Click the picture for a closer view] First, at the label "A", there is a part that looks to require a specific orientation. It has four leads and a directional marker on the part. You will note, however, that the board doesn't have a pin on or polarity indicator any where. We got it right - probably had an assembly drawing or illustration, but a pin one mark on the PCB would be a much more reliable method.

Second, at the label "B", you will notice that one of the pads for the little cap is in the middle of a real thick trace. The other pad has a very tiny trace connected to it. There may have been an impedance reason for doing this. I don't specifically know, but if not, you should avoid a layout like that with small parts. The thicker trace on one side can act like a heat sink and slow the paste melt on that side. Then when the solder paste on the other side melts first, surface tension can cause the part to pop up like a tombstone.

Duane Benson
EMI is mighty fine

Copper and Tombstoning

Earlier this week, I wrote about the effect soldermask can have on tombstoning and I referred back to two older posts on the subject of tombstoning. I also wanted to refer back to a third tombstoning-related post that talked about the effect that your copper can have on tombstoning, but I couldn't find the post. Well, apparently, I never actually wrote that post. I meant to but forgot and moved on to other subjects.

Any, way the point I wanted to reference related to the heat-sinking characteristic of copper. With small parts, uneven heating can lead to tombstoning. If one side of the solder paste melts before the other, the surface tension on the melted side can pull the part up.

In the first case illustrated here, (click image to enlarge) one side of the part has a small trace going to it. The other pad has a much wider trace. The eider trace can act as a heat sink and slow the melt of the paste on that side, causing the tombstoning.

In our second illustration, (click image to enlarge) an inner copper layer goes under one pad but not the other. As with the wide trace, the inner layer may sink heat away from that layer and keep the paste from melting at the same time on both sides.

It may not seem like much, but an uneven melt of just a very short time span can lead to tombstoning. That makes it a good practice, especially with small parts like 0402 and 0201 passives, to consider the thermal characteristics of your layout.

Duane Benson
Oh! ye'll take the high road and
I'll take the low road...