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- BOM (Bill of Material) sent to us in
a spreadsheet format is one piece of information that
is critical for both the quoting and assembly process.
BESTProto like most other companies doing PCB
assembly, we prefer to receive them in an MS Excel
format or something that we can convert to Excel.
As previously mentioned the BOM tells the assembler
the total number of components that need to be
placed on the board as well as how many different
types of components will be used on that particular
assembly. The number of parts will determine how
long the process will take and the different/unique
types helps determine the amount of work and time it
will take to "set-up" the machine or process.
The BOM usually has the information related to the
physical size and configuration of the part otherwise
referred to as the "package". Depending on the type of
package a given component comes in will again,
dictate the amount of work/time to place it on the
board. A simple resistor in a standard 06/03 package
as an example is one of the easiest and fastest
components to place however, if you reduce the
package size down to say an 02/01 then the time
required will likely be longer because of the accuracy
required to correctly place smaller parts is higher than
most larger parts.
Any parts requiring hand placement such as through-
hole connectors, switches, battery holders and the like
will require the boards go into an area often referred to
as "secondary assembly". Since this is a manual
process it's usually much slower and more labor
intensive. Again, a good BOM will tell the person
doing the quote most of what they need to know about
the component parts are to be used to assemble the
board.
Gerber Data contrary to popular belief is not
about how much baby food is sold by Gerber Products
Co. It is however the digital information that's used by
the board fab houses to produce the bare board. On
the assembly side it's used to give a graphic
illustration as to the relatively complexity and density of
the board design as well as the overall dimensions,
parts spacing and in some cases the panelization
scheme. Gerber data is also used to generate the
solder stencil which is used to apply solder paste in
the appropriate positions on the board.
CAD Data is information that comes under the
heading of "nice to have" during the quote process but
is essential to the board assembly process. It
contains the digital information that is used to
program the "Pick and Place" machine. Depending
on the software package, it may refer to a different file
name. Some familiar file names are Gen-CAD (.cad)
files, IPC (.ipc) files, 356 (.356) files, or other ASCII
format files
Solder Type should be specified based on
whether the board was designed to be leaded or lead-
free (RoHS compliant). You should also indicate
whether your assembly needs to be cleaned or if there
is any specific solder or solder paste that your
assembly requires. If you're not sure whether or not
the solder or paste that has been specified need to be
cleaned the person quoting the assembly will be likely
be able to help you determine that.
Test Information some form of testing is
always a good idea. When you're working on
Prototypes the test procedures often times haven't
been defined therefore you rely more heavily on visual
inspection. In the case of visual inspection you are
looking at part orientation and the quality of the solder
joint. In the case of BGA and QFN devices X-Ray
equipment is needed to verify that there are no shorts
underneath the device. Another tool that is used to
verify that the proper solder connection has taken
place is Endoscopic Inspection. This tool is used to
look under the sides of the device to verify that all the
solder joints look uniform. Any areas that look taller or
shorter than the others are likely to indicate that
something is amiss.
Other types of testing available are: Functional Test where you test the actual
function of the board to see that it performs as
designed. Automatic Optical Inspection (AOI) as the
name implies is an automated way to perform a visual
inspection of the board assembly. AOI works well for
small volumes where the products are produced
repeatedly. To get the best results the equipment
requires "teaching". In some cases it will make "false
calls" where it indicates a problem when in fact
everything is fine. It is also used in conjunction with
other forms of testing to ensure the board has been
tested thoroughly. Flying Probe Testing is a good candidate for
prototypes and smaller volume board builds. The
cost to set up and do the actual testing makes more
sense and the test coverage is generally quite good
depending on the design. In Circuit Testing (ICT) is a very effective test
method and is the test of choice for high volume
applications. It's relatively fast and the test coverage
tends to be very good however the cost of having a
fixture made and a test program written normally isn't
justified in smaller volume applications.
Special Requirements List any special
requirements such as a lead-free BGA on a leaded
assembly, "Do Not Populate" (DNP) areas or
components, stacked components, jumpers, cuts and
straps. It's always better to know these types of
requirement upfront as it may dictate.
The more information the better rule applies
to both the board quote and assembly process. Any
guesswork will likely increase the cost and create
delays. The more information you can provide will
help determine how the board will be built and will
give you the best chance to save time, money and
increase the overall yield.
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