Getting The Most Out Of Your

Printed Circuit Board (PCB) Design

Services Bureau

by Bob Chandler
May 2004

In today’s design economy, the large internal PCB layout department is getting rare.

Engineers are being driven whether they like it or not, to use outside services. To avoid the more costly disasters, you need to understand the dynamics of remote layout. We will discuss 7 aspects of contract design:

     1.   Service Bureau
2.   Communication
3.   Design Review Process
4.   Quality Control
5.   Schedules
6.   Budgets
7.   Change Control

1.  Service Bureau Spectrum
All layout companies can be plotted somewhere along the service bureau spectrum. On the far end of the spectrum is the “auto route” service company. This company will take your spectra file and process it through the auto router. You get the results, complete or not. At the other end of the spectrum is the project management and engineering house. This company can take your concepts and do all of the engineering, part selection, layout, prototype, testing, and production. This company will take most of your worries away, but you will also lose control of the product.

Most companies are located somewhere between the extremes, you will need to understand what your needs are, then select the company that best serves your needs.

If you company has a large, in-house engineering department with a complete support staff, you may only need occasional routing support or you might send out the boards for minor changes and “product life” support.

Relying upon a service bureau for all of your layout needs is not a bad thing. Smaller companies and even larger ones send out most if not all of their layouts. When you are making this much of an intellectual product investment in a company. You want to be sure to select the right company.

2.  Communication

  • Designer/Engineer Communications
    •    Layout Guidelines and Technical Issues
  • Customer and Vendor Communications Documentation Practices
    •    File structure
  • Business Communications
    •    Budgets
    •    Schedules
    •    Deliverables
  • In house Design Group

3.  Design Review Process
Each company whether they mean to or not, develops a process to review the boards. Some companies are more formal with the process that others. However, it is done, there are always three major areas for review.

a.) Placement Review – once all of the library footprints have been identified and built, and parts have been placed on the board, the most important of all reviews takes place.

During the placement review, you will need to verify 3 items:

  • library footprints
  • electrical constraints requirements and
  • the manufacturability of the board, or dfm review.

The review of the library footprints is very important. Be sure that the connector pin numbers are correct. That the soft-23 packages are pinned properly, that the diodes are not backwards. The electrical constraints review looks for things such as analog ground pins within the digital ground areas. If the buss needs to be daisy chained, is it in a logical order? Are the decoupling caps close to the power pins?

The DFM Review takes into account the mechanical interface requirements and the manufacturing processes, required to build the board. Included in the mechanical interface are connector locations, mounting holes, and height requirements. The DFM Review will be different, depending upon the total number of boards to be built, and the technology to be used.

Once the placement review is complete, and all interested parties have had a chance to comment on the board, it is then time to start routing. Please do not be afraid to let every department that has a concern look at the placement. It is much cheaper and faster to make changes here than it is to change the board after it has been fabbed.

b.) Routing Review – is normally broken into 3 reviews:

  • Critical route review
  • Power/Ground Review
  • Full Route Review

These reviews do not need to be done in any particular order, and they will change depending upon the electrical requirements of the board. The important thing to get clear at the start is: How much review and when, will be required? If you are expecting to see and approve all of the clocks and the different pairs before general routing, but the vendor is planning to do one complete auto-router pass, then both will be unhappy.

When reviewing the power and grounds, keep in mind the source of the voltages. Are there enough pins or vias to handle the current?

The final review takes place after the routing is complete. Documentation items such as notes, drawing numbers, revision markings, silkscreen and assembly clean up are addressed here. It is often helpful to have a preliminary documentation review while the route is in process. This will save time and frustration at the end, when you are rushing to get the board out.

Once the board has been through fabrication, assembly and test, it is a great idea to get back with the vendor and discuss what went well and what didn’t go so well.

4.  Quality Control
Everyone knows what a good quality design looks like. Everyone knows what the best industry practices are. In short, every one knows what QUALITY is. The only problem is that quality, like beauty, is in the eye of the beholder.

Every design service company wants to deliver quality. Every design service company wants to do a good job, but this is not always possible if you have not shared with the vendor what quality is for you.

Not all companies have written documentation standards. Some companies have very exacting requirements for drawings, notes, file names, formats, and styles. Other Companies have no standards at all. At either end of the spectrum that you find your company, you can do a few things to insure the quality you get from your vendor is the quality you expect.

First, tell the vendor what quality is to you. Sample drawings are great for showing the vendor what your formats, notes and drawings look like. This does not need to be anything extravagant. If your Company has ever done a board before, can you get a print of the documents? Even better if you can get a copy of the database, this will tell the vendor how to structure the new database to match your standards.

DFM – or design for manufacturing manuals can be very helpful for a service company. Most large companies and some smaller ones have these. If your company does, getting a copy of it to your vendor will make your project run much smoother.

Also, most contract manufacturers have their own standards, and are eager to get involved early in the design process to make production easier.

Another thing you can do to improve quality from your vendor is to obtain regular updates from the vendor. Getting daily prints of copies of the database lets you see what they are doing early, and gives you a great opportunity to correct it before it becomes a problem.

The last thing I want to mention about quality is the most important- FEEDBACK.

Don’t be afraid to give positive and negative feedback to your vendor. They want to do a good job so that they can get more work from you. It does not help them or you to simply crumble about the poor quality you got from your vendor. Be pro- active and let them know what went well, and where they could improve.

5.  Schedules
It is a well established fact that every project is, by definition, behind schedule. By the time the board makes it into layout, it is late. That said, there are three items that you will want to observe in regards to schedules:

  • Schedule slips
  • Honest Communication
  • Progress

Before you can have a schedule slip, you need to have a schedule, when you transfer the data across to your vendor, be specific about your schedule. When will the library parts be complete?

When will the placement be done?
Critical routes and complete routes are expected when?

One of the biggest mistakes a client makes is to assume that the vendor understands the schedule requirements. The vendor has incentive to get it done quickly, in order to get paid. This does not ensure that it will relieve the priority and resources you expect. You need to be sure that you have to pre-set schedules and milestones with which to monitor the progress. That way, you can see schedules slips before they become a problem. If you were supposed to have a completed placement to you on Tuesday, and it is now Friday, you might have a schedule problem.

Which brings us to the next topic – Honest Communication vis a vis schedule slips.

Do not use a PCB Design Service Bureau who continually missed deadlines that they have promised. Some PCB Design Service Companies will tell you that they can meet and un-realistic deadline, knowing that once you commit to the design, they will complete it at their own pace, because they know that you will not pull the project after the deadline has passed.

PCB Design Service Bureaus who may have a little higher in price, but who meet the schedules, can be cheaper in the long run.
Not all schedule slips are the fault of the vendor either. If you are on your own ninth net list in four days, maybe you are the cause of the schedule slip, do not be too quick to blame the vendor. Blame does not help anybody and should only be used when deciding to use the vendor again or not. The most important thing about a schedule slip is “what do we do about it?” By finding the problem early, more recourse can be brought to be on the problem.

Another item: Be REALISTIC about your schedule and the costs if they are missed.

I worked with one company who had a board that “had to be out of July 1, or it would be big trouble for the Company!” We worked three shifts to get the board ready for fabrication. It went to the client for final review on June 28. July 1 came and went. July 10 came and client was still reviewing, and would get back to us. August, September, minor changes…. November, December, the board finally went to fabrication. In mid January. Where was the big trouble? How likely is that we will work with two shifts, let alone there for this client again? If you are not honest with the PCB Design Service Company, you can not expect miracles from them when you really do need it.

The last item dealing with schedules is progress. How are they doing? If you do not watch how it is going, you do not know when they are in trouble. It does not take much to get a daily status report. It may be a complete database for review, or just a few lines in an e-mail, explaining what is happening. You should never be surprised by a missed schedule. It is simply too easy to pick up the phone, or shoot off an e-mail to get a status.

6.  Budgets

7.  Change Control