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Lesson 3 - PCB Design



Main Entry: 1sche·mat·ic
Pronunciation: ski-'ma-tik
Function: adjective
Etymology: New Latin schematicus, from Greek schEmat-, schEma
Date: 1701
: of or relating to a scheme or schema


From Merriam-Webster’s on-line dictionary.




Schematic Symbol Recognition:

See if you can classify the following product categories:

Design Parameters

bulletPre-layout Design Reviews

Before design activity can begin, the 'ground work' must be laid to establish the form, fit and function of any printed circuit board. Successful PCB design happens when Marketing, Program Management, Engineering, Manufacturing or Production Control, Test and Integration,  Material Control and Quality Assurance work in concert toward a design that functions as the customer requires, with a reasonable performance margin, for a reliable product life cycle that the customer needs..., and for the price that the customer will pay and a profit margin that makes the company healthy and successful.

An overall approach is needed to support the entire program, not just the needs of any one department or small group of individuals. Having a board work in the Engineering Lab is not proof of a manufacturable product, it is only proof of concept... not designed to the rules required to make a reliable, manufacturable, testable, and deliverable product.

  Before layout, the team members from Planning, Engineering, Manufacturing, Material Control, QA, and Test need to be consulted and given a opportunity to review the 'potential product' to give their valuable input as early in the design process as is practical to make sure that all disciplines are informed and their design considerations

are evaluated and planned for, in the initial stages of the design. This does not mean that an engineer can't prototype his or her circuit before these meetings, but the meetings are essential if you are to produce the product in a cost effective manner.

  The PCB Designer must be prepared by knowing the needs of the different departments involved and being sensitive to those needs in developing the printed circuit design. 
The following are examples of questions that could be asked by the PCB designer at the Pre-Layout design review meeting.
To be discussed :

   When do we need the design done (how about a schedule)? 

   What is our cost target for the PCB? 

   How many units do we expect to ship per year? per month? per quarter?

   How are we going to test the PCB? In-house? Outside?

   What are the customer requirements? (this will be Marketing's info)

   What is the expected life of the product?

   What Logic families are we dealing with?

   What frequencies will we be dealing with?

   What limitations do we have electrically?

   What limitations do we have mechanically?

   What limitations do we have environmentally?

   Are there any high current devices to be concerned about? 

   If so, what currents and voltages?

   What sort of thermal challenges will the design encounter?

   How will the board be mounted?

   What sort of connectors will be needed?

   What sort of user interfaces (panels, switches) will there be?

   What sort of indicators or displays will be required?

   What agency approvals will be required?

   What are the physical constraints for the PCB assembly

   What bus structures will be employed?

   Do we need impedance matching?

   Will we need stripline, micro-strip or special RF or Microwave technologies?

   What material(s) do we wish to use?

   Will the board(s) be solder coated? 

   How will the boards be assembled? 

   Will the boards be through hole? Surface mount?

   Do we have library parts for all the component footprints?

   Do we need data sheets for specific components to build those parts in the library?

   Etc., etc, etc...

  A proper review of the design parameters prior to beginning work on the PCB is paramount. If you don't have the review, you will most likely be doing the board over again...




Design Parameters

bullet Customer Specifications Marketing Requirements and Cost Targets 


YOU as the PCB Designer need to know what your marketing people already know from interfacing with your customer... What does your customer really want..., what will they pay for it and when do they want it delivered? With few exceptions, designs are really built for a customer... whether it's one company or an entire global consumer based market. The customer has a specific need, which you and your company hope to satisfy... in exchange for  dollars. This makes your customer happy, and keeps you and your fellow employees gainfully employed...  of course...- The American Dream... success.

Here are a few givens:

If you design something that your customer does not want... your customer will not pay for it... your company looses money,  you need a new job... 

If you design what your customer wants... but it costs too much or your purchasing department can't purchase the parts because of long lead times or obsolete parts..., your customer will not pay for it... your company looses money, you need a new job... 

If you design what your customer wants, but your manufacturing people can't build it....your company will not ship it, your customer will not pay for it... your company looses money, you need a new job... 

If you design what your customer wants, and your manufacturing people can build it....but your testing department can't test it....your company will not ship it.., your customer will not pay for it... your company looses money.., you need a new job... 

I think by now you get the picture... 

Ask your Product Manager, or Electronics Engineer to discuss the PCB design intent and end use of your product, from the Marketing Proposal, before beginning your layout. Find out what sort of environment your board will need to survive in.
Will it be expected to work at altitude, under vibration, at extreme temperature and/or humidity? Will it need to work at very High frequencies? or High Currents? or low leakage? What about EMI or Radiated Electromagnetic Interference..., are there sensitive amplifiers that need shielding? or mixed signal issues? Just putting components on a PCB and hooking them up does not necessarily guarantee that your board will be a success. You need to apply the right combination of features to take advantage of all the tools at your disposal to succeed. 

By the time the Electronics Engineer is ready to give you a schematic diagram and a parts list he/she should already have thought about these issues..... (but don't count on it...) If you prompt him/her with the appropriate questions they usually will get the answers for you, which will help to make your design a successful one.






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