Screaming Circuits: December 2007


5 Basic Steps to a Successful PCB Layout - Part 4

This is the fourth of five parts from guest blogger, Stilwell Baker, covering successful PCB layout. Last week they covered layer count.

Know your PCB Fabricator: Every PCB fabricator has their preferred design guideline

  • Be sure the fabricator that you are using is capable of producing the type(technology) of design that you are creating. For example, if the fabricator you've chosen can't do the rigid flex that you need, you will need to find a set of cables and connectors or go find a fabricator that can build rigid flex.
  • Work with the fabricator to build a stackup. This is especially important on many of the high speed designs. Although it is easy to download one of the free impedance calculators out there and punch in what you think are the correct numbers. Give you fabricator a call and verify the number that you are assuming. In all of cases they may be able to work out a stackup for you.
  • Get there fabricators design capabilities and design to their standard where possible. Know what advanced means.
  • If you are creating a design for R&D at a prototype fabricator and plan to send the completed R&D design to a different volume fabricator, be sure that the R&D design has been created to accommodate either fabricators guideline

Next week: Know your assembler

Random PCB Assembly Questions

Aensoldering120 We have a lot of information on our website. Hopefully it's mostly all useful by somebody, but even if it is, there's still a lot to wade through just to find the two or three factoids you are looking for. Sometimes I can't even find what I'm looking for. We do have the little search box up in the top right, but it doesn't always work like you would think.

For example: we received an email the other day asking what kind of flux we use for lead free. (We use no clean) I know that we use no clean solder and solder paste, but just for kicks, I put the word "flux" into the search box. I got nothin.

On our capabilities page, we do say that we use "no-clean" but we say it in reference to solder type, not flux. As far as I could see, we didn't use the word "flux" anywhere on our web site. Well, that's not so smart. Somebody want to fix that for me?

Duane Benson
Must flux website

Soldermask, Tombstoning and Solder balls

Soldermask_tombstoning_for_blog 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!

5 Basic Steps to a Successful PCB Layout - Part 3

This is the third of five parts from guest blogger, Stilwell Baker, covering successful PCB layout. Last week they covered parts libraries.

Be careful with layer count: Additional layers can speed routing, but if your end product will need fewer layers use caution. A working 6 layer prototype may not easily translate into a 4 layer design.

  • Look carefully at connector and HDI escape routes to see if you have leveraged layers that will not be there in the reduced layer count design. If you have is there space on a different layer? Will you have to modify planes to accommodate these traces?
  • On high speed designs the layer reductions can have dramatic effects on return paths. Added length, added ground or power splits, …

Here at Screaming Circuits, we see everything from single layer and up. I think the highest layer count has to have been 24 or something. In addition to the notes from the Stilwell Baker folks above, lots of layers make the fab more expensive and the assembly more of a challenge. The board will be more sensitive to the thermal reflow profile and will have a lower tolerance for rework.

Next week: Know your PCb fabricator

A Couple of Interestingly Tiny Parts

Yeah, I'm kind of a broken record on this subject. But, you have to expect that given where I work and that I believe in what I do, I'm going to be such a broken record.

I ran across these two parts a while back while selecting components for a robot project 7mmx7mm_qfn_pic of mine. I had 4mmx4mm_qfn_maxbeen using 300mil wide DIP parts and some SOIC packages, but even though this isn't for a commercial venture, I wanted to reduce cost and size a bit.

In the high-volume professional world, this is standard operating procedure but it's also spilling down to the hobby market and the professional prototyping world. Each designer, professional or otherwise, that forgoes a big thru-hole part for something tiny like this, give part makers less incentive to ever build those big packages.

Duane Benson
Lead puppies aren't much fun

Top-Ten Reasons to Ship Us Your PCBs in December

Yes, Christmas is near, as is New Years. Some companies shut down for the holiday week. Some don't. Some rush to spend remaining budget dollars before the end of the year, while some don't. Regardless, there are quite a number of reasons to send your prototype off to Screaming Circuits for assembly before the end of the year.

#10. You can come back on January 2nd to a clean desk.

#F. Or you can come back on January 2nd to a freshly assembled PCB, ready to be probed by that big expensive scope you convinced your boss to let you buy.

#E. Get those obnoxious marketing geeks off your back: You can show them the assembled boards and say: "See, your stupid prototype will be ready for the CES show in January. Now quit bugging me so I can do my job."

#D. As suggested in the opening to this post, you can squeak out some more budget dollars before the close of your fiscal year.

#C. Skiing! Send us your kit, sneak out early and hit the slopes for a long weekend. Skis or boards?

#B. Surprise the family. Start with #C, but instead of running off to the slopes with your ski bum buddies, build a snowman with the kids. Heck, build a whole snow family. Maybe do a "Calvin & Hobbes"

#A. Okay, if you live in Oregon, you probably won't be able to do the snowman thing, but you can leave early and do some singing in the rain! Singing in the cold, dreary, miserable, incessant rain that won't stop. It just keeps raining and raining and it never, ever stops...

#9. By getting the boards assembled now, you will know how many extra FPGAs you will have left over and you can take those spares and hang them on your tree as decorations.

#8. See #A, above. Get the proto assembly off your desk so you can silently wallow in your SADs without getting further behind schedule. Then get facilities to buy you some of those outdoor spectrum florecent lights that are supposed to help.

#7. That other project team that always seems to be on schedule... Get a jump on them with a 24hour turn here. You can tell them about us later.

#6. It's fun and entertaining.

#5. Stick it to your competition by beating them to launch.

#4. Help with your Christmas shopping - get the assembled boards back and wrap them up as presents to put under your tree. Okay that one's pretty lame, I admit, but it's hard to think up 10b16 things for a top-ten list and not have a few lame ones. #6 was pretty lame too.

#3. Sleep. Send us the project to assemble and stop stressing. You need some good sleep. Holidays are stressful enough without an unassembled proto hanging over your head too.

#2. Don't forget turn-key. have us order the parts and boards for you and save yourself even more time.

#1. And the number one reason to send us your board for assembly before the end of the year: It will make us work longer hours and cut into our holiday time. That's a little bit of payback for all of those annoying marketing message we keep sending your way.

Duane Benson
And a very happy holiday to you too!

5 Basic Steps to a Successful PCB Layout - Part 2

This is the second of five parts from guest blogger, Stilwell Baker, covering successful PCB layout. Last week they covered parts libraries.

Mechanical constraints matter: Trace density, layer count, and physical size all matter, but the parts still need to fit on the board.

  • Connector locations and orientations can quickly limit placement and routing flexibility
  • Finish the mechanical specification before moving into the placement and route of a design. Changes to mechanical specifications during placement and routing can significantly increase the amount of time in layout.
  • Be sure to budget extra space for mechanical interfaces that plug into the board.
  • You’ve checked your part spacing and and connector locations in the x and y direction, now it is time to make sure that you have accounted for component heights in the z direction. Check for external mechanical interferences. For instance the curvature of the case that the board is mounting in, the motorized arm that moves along the adjacent interface, ….. Sometimes an IDF or DXF transfer to check your design in the larger mechanical world is the best thing.

Next week: Be careful with layer count.

Christmas and New Years shutdown Schedule

Of_schem2 We will be closed on December 24th and 25th for Christmas and again on January 1st for New Years. This means that those days won't be counted toward your turn times. For example, if you have asked for a 48 hour turn time and we receive your kit on the afternoon of the 21st, your 48 hour clock will start on Wednesday, the 26th.

We apologize for any inconvenience and wish you a happy holiday. Eat well, drive or fly safe and enjoy your friends and family.

Duane Benson

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:Pict0043

  • 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

5 Basic Steps to a Successful PCB Layout - Part 1

Sbi_logo_for_sc_1107wideStilwell Baker is our favorite PCB design company in the Northwest. They provide PCB layout, Library Management, DfX engineering and Component engineering. With 20 or more designers and many years in the business, they have a wealth of experience. Joe Zaccari and the team at Stilwell Baker have generously offered to share some of that expertise in a five-part blog post.

Today, he's starting with advice on parts libraries. Check back weekly for the next four parts of the series:

  1. Know your library (today)
  2. Mechanical constraints matter
  3. Be careful with layer count
  4. Know your PCB fabricator
  5. Know your assembler

Taking the time to review a design prior to layout and resolve issues early results in lower costs and faster time to market. No rocket science here, just the old, “Find time to do it right the first time, not time (and $$$) to do it over”, lesson rearing it’s ugly head again. Many times PCB layout becomes the sponge that absorbs all of the schedule slips in the R&D process. Unfortunately, a great design, laid out poorly can be lethal to a product.

Know your Library: Part libraries are the single biggest source of problems with PCB layout.

  • Part symbols on the schematic and part footprints in the Layout need to be verified and checked somewhere in your library process. Without this check, a defective part may find its way on to more designs before it is actually physicalized and tested. These types of part defects can be as simple as a not having the correct polarity to having the schematic symbol referencing the wrong footprint. Some defects will be easy to see when the part is being assembled, while others won’t appear until after some hair pulling and a few hours on the lab bench.
  • Try to stick with industry standard part package types when possible. This will help the designer, fabricator, and the assembler. Trying to figure out how to create a footprint for something special and then trying to get it fabricated and assembled, may not be the best idea for designs on the fast track to market or near the production phase of your R&D.
  • If you need to get creative and use a new part type or technology. It may be advantageous to include the fabricator and assembler along with your layout resource early on in the design process.

Next week: Mechanical constraints.

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