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DESIGN GUIDELINES FOR MANUFACTURING CIRCUIT CARD ASSEMBLIES
Introduction:
Kuchera Industries Inc. has created this document as a
tool to optimize production and minimize potential errors. The use of
this document will improve on time delivery and help to assure quality
based upon the experiences and expertise of our dedicated employees.
I. Documentation
Every kit issued to Kuchera Industries, Inc. goes through a documented
assembly process. In order to start this process, Kuchera Industries
requires the following documentation supplied for each kit:
A. Purchase Order
B. Photo plotting Data Files (Gerber data)
C. Assembly Files
D. Assembly Drawings
E. Bill of Materials
Purchase Order
In order for Kuchera Industries to start an assembly the first thing
needed is a purchase order. Kuchera Industries uses a business system
that requires a purchase order be entered before the assembly process
can begin. The purchase order needs to contain the following: bill
to, ship to, price, revision level to build assembly to ecn’s
pertaining to kit workmanship spec., quantity to be assembled, assembly
number, method of shipment for final product and any other special
instructions (Kuchera’s quote and the purchase order are the
two documents that govern any order). The purchase order can be faxed,
included in the kit, or electronically sent to the following address:
shuty@kuchera.com or daloni@kuchera.com. Kuchera Industries recommends
that the purchase order be faxed and a copy included with the kit.
Photo plotting Files
The common name for these files is Gerber data or Gerber files. The
main purpose for these files is so solder paste stencils can be fabricated;
these stencils are used for the application of solder paste during
the surface mount assembly process.
CAD Files
In order to assemble a PCB, information about the components used
on the PCB and their location must be communicated to Kuchera Industries.
This information includes reference designators, (R1, U1, C1, etc.),
component description (1206, TANTB, SOIC8, etc.), part numbers, component
value (10k, .1uf, etc), x-y location, orientation and which side of
the PCB (top, bottom).
Assembly Drawings
An updated and accurate assembly drawing with current Rev level contains
instructions on how to assemble the PCB. It contains information that
is not present in the electronic pick and place file, such as hardware
that needs to be mounted or special assembly instructions. This document
is the main guide that Kuchera Industries follows for the assembly
of the PCB. Assembly drawings should be transmitted with the other
electronic files. Kuchera Industries also requires that a hard copy
be included in the kit.
Bill of Materials
Bill of Materials contains a complete listing of all the parts, labels,
and other items needed to build a PCB assembly. They contain the customer
specific part number, manufacturing part number, part reference designator,
revision level and other information needed to assemble the PCB. Kuchera
Industries requires that the Bill of Materials be stored and transmitted
with the other electronic files (in Excel format) and a hard copy
is included with the kit.
A typical collection of the electronic files that need to be sent
to Kuchera Industries for the assembly process:
BOM.xls - Bill of material in Excel format
Readme.txt read me file describing each file sent
Applist.apt - aperture list for Gerber layers
Pastetop.gbr - solder paste Gerber file for top side
Pastebtm.gbr - solder paste Gerber file for bottom side
Silktop.gbr - silkscreen Gerber file for top side
Silkbtm.gbr - silkscreen Gerber file for bottom side
xxxxx.asy dwg. - assembly drawing
xxxxx.fab - fabrication drawing
xxxxx.smt - x-y location of components (pick and place file)
ASCII -Mentor Graphics Neutral File
-solder mask files top & bottom
To assure timely delivery of all assemblies the documentation should
be sent in advance of the kit. This will provide time for pre-production
planning and ordering of necessary tooling (solder stencil and wave
pallets).
II. Silkscreen / Masking
A. All components on the board should have a silkscreen outline.
B. Reference designators should accompany each component on the board;
they should be numbered in logical, systematic order. The common methodology
is to start at one corner and number in rows across the board. Different
types of components have different alphanumeric designators, (i.e.
U1 (ic’s), R1 (resistors), C1 (capacitors)) the important point
is to create a numbering scheme so any part can be found quickly by
reference designator.
C. All connectors, edge-fingers, headers and similar type connection
devices should have a “1” silkscreened at pin 1, and the
last pin should have its corresponding pin number silkscreened also.
D. Polarized components should have a + silkscreened outside of their
component outline at the positive lead pad.
E. Silkscreen can not flow over through holes or pads, which are
to be soldered.
F. PLCC’s, PQFP’s and any other IC’s should have their
pin 1 silkscreened adjacent to the appropriate pad.
G. PCB should have “tented” vias (covered with solder mask)
on both top and bottom sides of the board.
III. Component Spacing
Component spacing requirements have a direct effect on the density
of a PCB design and, therefore, the manufacturing of the board. The
spacing requirements need to be well specified and communicated to
the PCB CAD designers, including relative orientations of the packages
with corresponding minimum spacing rules. Spacing rules in general
refer to the land pad to land pad spacing. Any component body overhang
needs to be accounted for in the land pad spacing values (reference
IPC-D-275, 2221, 2222, 2223, 2224, 2225 for Industry Standard Specifications
for board design). There are a number of considerations in determing
the required component spacing.
A. Rework
B. Process Considerations (i.e., shadowing)
C. Inspection
D. Placement equipment
REWORK
Rework spacing considerations include the ability to access solder
joints with the rework tools and equipment, if necessary, without
damaging the assembly or exceeding the maximum adjacent component
temperature.
PROCESS CONSIDERATION (i.e., shadowing)
Process considerations include minimizing shadowing of the solder
joints that could lead to incomplete soldering during the wave solder
process. Consideration should be taken to place only passive components
(caps, resistors) on the bottom side of the board.
INSPECTION
Inspection considerations concern the ability to inspect the solder
joint fillets to assure adequate solder volume and reflow conditions.
Do not place any components on the opposite side of ball grid arrays.
This interferes with x-ray inspection.
PLACEMENT EQUIPMENT
Placement equipment considerations include assuring that the assembly
step sequence does not allow components to be placed that could interfere
with later placement operations.
IV. Panelization
A. Panelization refers to creating a multiple image panel for PC
assembly; consult Kuchera Industries on a case-by-case basis for size
limitations and suggestions. The method of depanelization is a critical
decision. The PCB’s are more stable, ridged, and wave soldering
is easier when panels are scored with a v-cut. Kuchera Industries
is capable of handling all types but prefers a panel that is scored
with a v-cut. Leave at least ¼ inch of space around all edges
of the board (primarily for handling and wave soldering purposes).
If the CCA itself cannot be designed with a ¼ inch space, please
have your PWB manufacturer build scored edges around the PWB.
V. Fiducials / Tooling Holes
A. Fiducials are needed on the PCB’s to permit vision alignment
of the stencil components to the board. There are two (2) types of
fiducials: global and local.
B. Global fiducials are placed in at least three corners of the board
and they should be circular pad free of allowing independent x- and
y- axis distortion correction.solder mask and range from .025”
to .060” in diameter. Three fiducials provide better alignment
by
C. Local fiducials are used to eliminate component placement variation
due to localized PCB pattern distortion. Fine pitch components, high
pin count components, BGA, and micro BGA’s need local fiducial
points. Small parts usually only need one fiducial point, while large
fine pitch, high pin count components, BGA, and micro BGA’s should
have two fiducials.
D. Tooling holes are mainly used on some of the stencil machines
at Kuchera Industries for tooling pin registration. Tooling holes,
usually two, are on the order of 0.115” to 0.125” in diameter
and placed 0.197” to 0.394” from the edge.
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