5. Details of Part Feeding Attributes
While the software required to compile a BOM into a set of tray definitions becomes complex the attribute set is fairly manageable. For each in house part number 4 lines of text have been setup in a Parts & Vendors database. These lines define:

• PCB Decal and Package Type
• Assembly ID and Placement Offsets
• Part Feeding Information
• Part Storage and Bin Location Data
While PowerPCB attributes can contain this data Index Design favor the uses of database techniques as this simplifies the writing of processing software.

The first line is used to define the PowerPCB decals associated with Part Number. If a single string exists (as in the above example) then the decal for PCB design and for placement verification have the same name.

Line 2 (Asm_ID RXY) identifies the assembly machine ID for this component along with the Rotation, X and Y offsets. These offsets are used to translate from PCB design files to assembly machine formats. MyData machines use Unix based file systems and the legal
character set is small. A lookup table is constructed from the Parts & Vendors data as to allow third parties to setup and maintain their own translation tables.

Line 3 (Part Feed) identifies this part as being feed in a tray using a 0603 hole size that is 050 mils deep. The part is located in the tray at 0 degrees. Code in the BOM to tray “Compiler” uses this line to place the part into an inventory of predefined trays. Additional code takes the hole size name (0603 in this case) and drives a laser cutter that creates cardboard component trays. Cardboard trays are very useful as they are very space
efficient and they make it easy to visually verify that all parts for a job exist and are ready for placement. The hole depth field allows machined tray holes to be adjusted to match component height. Having small parts in controlled depth holes simplifies the packaging
of loaded trays and simplifies transport.

Line 4 drives component bin marking and locating functions. When gathering material for prototype assembly a significant amount of time is spent locating parts. Index Designs has set up a system where standard resistor and capacitors are stored in a loose form. It is much faster to reach for a small marked “Pill Box” and extract two or three loose parts than it is to locate a reel and extract 2 parts from tape.

Locating parts from inventory is a significant task. While reels are the most common form of individual part storage reels are difficult to store in an easy to locate and access manner. Index designs strips down a portion of its common reel components into pill boxes and sorts these into carriers based on size and type. A single carrier might carry 0603 capacitors while another might carry diodes.

The goal is to reduce the time it takes to load components for a job. Tray loading from loose parts in pill boxes averages about 350 parts per hour or about $0.09 per part. Loading of tape strips results in a loading rate of 2000 parts per hour. A medium sized board might contain 250 parts, which results in a worstcase setup of $18.00 per board. The threshold for moving from loose to tape strips is about 10 or 15 parts.

The BOM Compiler arranges the parts into trays such that loading errors are minimized. Experience shows that most part errors are generated as components are selected from storage. Bar Codes have been added to storage container bottoms to allow software verification of the selected component. In addition to bar codes laser cut trays are marked with component identifiers. While the loading of trays is tedious it is actually much faster than loading bare boards and the loading of reels is also a very tedious process. The goal is to minimize setup time. Whichever scheme prepares the assembly machine fastest is preferred.

Small parts typically load at 400 pieces per hour. Since all parts of one type are loaded at one time in a single row stuffing errors are practically eliminated. The small shallow holes in the tray surface allow parts to “Click In Place” which greatly speeds loading.

6. Schematic to PCB Tools
Once an external database is used to manage part placement it is relatively simple to expand this scheme to include Part Number to PCB Decal assignment. Index Designs provides PCB design services for a number of customers who use a variety of schematic
capture tools. In these schematics it is very common to find passive components with VALUES defined and some parts with incomplete manufacturer part numbers. Prior to PCB design the schematic items must be linked to PCB package styles as defined in the Index Designs library. Once all schematic items are mapped to physical packages the PowerPCB netlist is modified with the package assignment from the database. The resulting netlist is simply imported into PowerPCB.

The advantage to this scheme is that the schematic need not be changed to modify decal assignments and additional components (such as mounting holes) are easily added. Part value and reference designators from the original schematic are retained and BOM
compare function can detect changes in values, reference designator, pick and place setup, part location, rotation or side. These checks are very useful in detecting changes between revs. In many organizations a schematic BOM drives the generation of various supporting documents. The ability to detect and report a BOM change simplifies the task of
updating these documents and improves their accuracy.

“attributes” into schematic libraries is eliminated. Only a single resistor symbol is required in the schematic symbol library. Design engineers can concentrate on schematic Connectivity” with “BOM Annotation” saved as a final process.

The use of a reference schematic from vendors or schematics from consultants is simplified since write access to the schematic is not required. Consultants often have their own schematic tools and libraries andmost library data servers are not accessible through
company firewalls. Given the wide acceptance of the PowerPCB netlist format it is relatively simple to accept input from most any schematic capture program and map the designer part requirements to existing inventory parts.