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A Best Practices Design Audit
April 9, 2008 |Estimated reading time: 5 minutes
So your design is ready for fabrication - or so you think! How confident are you that your board is ready for fabrication and assembly? Many designers face this uncertainty as the deadline for fabrication approaches. What tricks can you use to assure your design is truly ready?
Let's take an inventory of our design process and review what makes a good design great. First, what details were considered, discussed and finally implemented to make this design?
There are basics to consider:
- Mechanical outlines and hardware positioning, as well as connector constraints
- Electrical considerations - differential pairs, RF details, digital matching, power issues, crosstalk
- Layer stackups and board fabric choices
- Special vias
Next there are special considerations:
- Scheduling
- Cost-adders
Lastly, designers must ask the question, "Are the overall constraints beyond current technologies?"
In most cases, the designer has no control over mechanical decisions. The connector placement and outline for the boards are defined by the peripheral hardware so the boards must fit within the defined package. But a good designer will ask at the start what is fixed or predefined and what can be adjusted. Often one dimension is set, but the board can be made longer in another direction with low impact to the packaging. If you don't ask for it, how will you ever know IF you can have more area within which to design?
Details, Details
This is also the right time to verify pin numbering and placement of any connectors or interconnect devices. Too many designs require additional revisions due to a designer's incorrect understanding of pin-outs and alignments for connectors. With the finer pitch used in many designs, you, the design expert, must require written documentation showing keyed locations and pin numbers on interconnect devices for use in verifying accurate placements.
Electrical considerations can cause dilemmas for the designer as the requirements for one constraint may be at cross purposes with another. For example, engineers often set specific rules for differential pairs within the schematic. This same effort may require crosstalk to be minimized, but what if the engineer wants a board with fewer layers than will allow for proper plane design for good crosstalk control?
Components represented as PCB decals can create some unusual constraints when considering the RF details for the design. Close proximity is often the desired layout, but size of coils, torroids and capacitors may not allow for placing components near each other. Height and 2D size must be analyzed to assure proper clearances. Matching signals may become important as the engineer implements finer-pitched devices. Understanding route restrictions for these before the traces are drawn is critical. Finally, power issues can be addressed in varying ways. Some solutions are more simplistic, such as using wide runs and larger vias to accommodate current and related signal issues.
The board's fabrication can easily make - or break- a good design. Designers need to understand the application of layer stackups and board fabric choices at their chosen vendor. Except in unique build applications, designers should consider the overall makeup of the board and its design so the board can be fabricated at more than one fabrication house. The use of unique technologies can and will greatly restrict the choices for fabrication. Consider using the standard via size wherever possible, and microvias only in areas where absolutely necessary. Are blind and/or buried vias REALLY necessary, or do they just make hooking up BGAs and similar small-pitch devices easier? Another consideration is using three or four choices as sizes for vias - using larger drills where the room allows and the smaller options just where necessary.
Using special vias is common on today's designs, but when reviewing several designs recently, I have found that the vias used did not need to be so small. Perhaps the choice by this designer's company to use a smaller call-out isn't appropriate for every design. This is worth discussing with your engineering team as a cost-saver.
Now let's tackle the unknowns, these special considerations that seem to catch designers off guard. Many of the decisions for fabrication and assembly are not considered early enough in the layout process. I have completed what I thought was a great design only to discover that the soldering flow was not the right one for our assembly contractor. Early buy-in from your team is critical to finishing designs on schedule. Some offshore sourcing can be beneficial; there seems to be less stringent rules about flow processes offshore.
On Schedule?
Scheduling has long been one of the design world's biggest demons. Technology needs time to evolve, but some designs require extra testing and evaluation, thus complicating the scheduling for a planned release. Understanding the goals for release at the start of the project can certainly help designers focus on accomplishing design completion in a timely manner. Engineering changes are a necessary part of the board's evolution, so good designers plan for these. But great designers refuse to let ECOs impact delivery dates.
How to accomplish this? Juggling is never easy, but designers with good communication skills can drive the schedule and completion favorably. Again, designers MUST ask questions and reach compromises with the whole team in order to be successful.
Cost-adders can be a designer's headache. The knowledge of current practices, especially in assembly, forms a critical path in design planning and completion. Using smaller vias, adding layers, demanding imbalanced stackups, and requiring unique fabrication materials all affect the total cost to manufacture. While not all these issues can be avoided, each should be challenged before moving ahead.
Lastly, designers must ask the question, "Are the overall constraints beyond current technologies?" Some engineers are less than sensitive to this dilemma. They feel if they can dream the circuit, the designers can design it and have it fabricated and assembled. Some constraints are still beyond standard fabrication and assembly capabilities at this time. One recent design submitted to me for evaluation ended up as a chip-on-board, because the technology in the size required did not fit the standard fabrication processes. Alternatives are important and great designers study trends and options, directing each design in a manner that's best for the technology. A great designer must be a perpetual student of the evolution of technology.
So, take inventory of each design as it's ready for release to fabrication and assembly. Stop, review, rethink and research.
That is what makes a good designer great!
Ruth Delker is a senior PCB designer for Cipher Systems in Beaverton, OR, and immediate past president of the Pacific Northwest Chapter of the Designers Council.