所需阅读权限 1PCB组装流程简述

Triton

(本文来源于pcdandf.com)
Printed  circuit  board  (PCB)  designers  focus  their  expertise
and  creative  abilities  on  transforming  product  ideas  into
product designs. Once  the design  is handed off  to  the PCB
assembler, the designer moves on to the next product design
challenge. However, assemblers must now focus their exper-
tise and creative abilities on transforming the product design
into tangible, real world devices that their companies can sell.
In  today’s  electronics  industry, where  functional  specializa-
tion, outsourcing, globalization and geographically dispersed
locations  are  the  norm,  it  is  not  uncommon  that  designers
typically have  little visibility  to, or knowledge of,  the  com-
plex processes involved in PCB assembly. This article presents
an  overview  of  typical  PCB  assembly  processes  in  order  to
provide designers with the context to the feedback that they
receive  from  assemblers, which  is  intended  to  improve  the
overall manufacturability of the product.

Manufacturing Line Processes
There  are  many  processes  that  a  PCB  might  go  through,
as  it  travels  down  the manufacturing  line.  These  processes
vary depending on  the  complexity of  the PCB,  the  types of
components  to be  fitted  to  the PCB and  the  target volumes
that production needs to meet. These processes may include;
screen print (stencil), paste inspection, glue deposit, automat-
ic  component  placement,  automatic  optical  component/pin
inspection (AOI), wave solder, reflow oven, manual compo-
nent placement, automatic X-Ray component/pin inspection
(AXI), flying probe test (FPT), in-Circuit test (ICT), boundary
Scan test (BST), functional test (FT).
These  processes  typically  have  a  predetermined  order,
though  various  factors  can  affect which  of  these  processes
is  used  in  individual  production  lines.  In  the  case  of  a
double-sided PCB, a second set of processes may be defined
that  is different  for each side of  the board. The above steps
fall  under  two  high-level  categories,  one  covering  the  steps
needed to assemble the components to the PCB, and the other
verifying  the  product  in  order  to  detect  defective  products
and to ensure product functionality.

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Assembly, Test and Inspection Programming
The majority of the equipment used on a PCB manufacturing
line needs to be programmed individually with the details of
what tasks they are required to perform. In most cases, this
involves  detailed  information  on  the  PCB  being  manufac-
tured  so  that  it  operates  correctly. Assemblers  usually  take
the  intelligent  layout files along with the manufacturing Bill
of Materials (BOM) file and use them to develop an overall
assembly, test and inspection strategy based on volumes and
equipment availability.
Bill of Materials Data
Designers  use  a  BOM  file  during  the  schematic  and  layout
phases,  though  in reality,  this  is rarely  the BOM  that  is used
in manufacturing. With  today’s worldwide PCB manufactur-
ing for both Original Equipment Manufacturers (OEMs) and
Electronic Manufacturing Services (EMS) companies, sourcing
local components near to the PCB assembly is the norm. There-
fore, a manufacturing BOM  is created and managed through
the assembler’s Enterprise Resource Planning  (ERP)  solution,
used to track the components and production schedules.
When  a  single  fabricated  PCB  supports more  than  one
variation of an assembled PCB, multiple BOM files will exist
to  describe  each  variant  of  the  PCB.  Here,  the  fabricated
PCB may be the same, but, due to component part changes,
may be assembled on different lines or on the same line with
minimal changes.

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Stencil Creation
Once the final land pattern and soldermask layers have been
created,  stencil  layers  can  either  be  created  from  the  paste
layers  or  derived  from  the  land  pattern  layers.  These  will
be  used  to  create  the  stencil  used  to  apply  solder  paste  to
the appropriate areas of  the board  in  the required quantity.
A  stencil  is  used  to  apply  the  paste  that will  be  used  in  a
reflow  line,  this  is  typically  the method  used  to  solder  the
components to the board. The stencil is a metal screen with
apertures cut in it to let the paste through to the board under-
neath, as the squeegee moves from one side of the stencil to
the other.
The  stencil  apertures  can  be  a  percentage  reduction  in
the  size of  the original copper pad, a  specific distance  inset
from  the copper pad or some  form of custom aperture  that
is not related to the copper pad underneath but more a func-
tion of the package that will be placed on the  land pattern.
Pin surface mount components usually have an aperture that
looks like a notched rectangle with notches pointing inward
toward each other.

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Assembly Equipment
Any  individual  component  on  the  board will  be  physically
placed by one machine during  the manufacturing process.
Therefore, decisions need to be made that determine which
machine will place each component onto the PCB to maxi-
mize through-put or to minimize line configuration chang-
es.  Line  balancing  is  the  technique  used  to  decide  which
machines will place which components. Usually, there may
be only one machine  that can physically place  the compo-
nent to the board; however, it is not unusual to have choices
for individual components, so decisions have to be made as
to which machine should ultimately place the component.
There are many factors that can affect machine choices.
For  highest  throughput,  the  line  should  be  balanced  to
reduce  the  slowest  machine,  or  bottleneck  process,  that
dictates the overall throughput of the line. In high mix envi-
ronments, where  throughput  is  less of  an  issue, minimum
changeover may  be more  of  a  factor. Here,  the  line  con-
figuration is set up to reduce the number of machine setup
processes  that  need  to  take  place when multiple  products
are being run down the same line. Moving a single compo-
nent  from  one machine  to  another  can  have  a  significant
effect on  the  individual machine optimization  and overall
line optimization.
From  a  design  perspective,  the  component-to-compo-
nent  clearances may differ between  two different  types of
placement machines. Therefore,  if  the part  is being placed
by one machine, a close clearance may be possible compared
to the clearance with another type of machine. Some parts
can be supplied in different packages for the same electrical
functionality, but  selecting one package over another may
affect the types of machine that can place the specific part
and its associated yield during manufacture.
Once  components  are  allocated  to  a  specific machine
within  the  line, machine-specific optimization  can be used
to  improve performance within  the  line. This  is known as
machine optimization, and combining line optimization and
machine  optimization  multiple  times  to  achieve  a  better
solution is called iterative line balancing.
Given  these  decisions,  designers  can  understand  why
component and package  selection  can affect an assembler’s
ability to create optimized machine configurations for PCBs.

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Inspection Equipment
There are four categories of inspection equipment for manu-
facturing defect detection: Paste Automated Optical  Inspec-
tion (AOI), Pre-reflow AOI, Post-reflow AOI and Automated
X-Ray Inspection (AXI).
Paste  AOI  is  used  to  detect  any  issues  occurring  after
the paste has been laid on the PCB. At this point, very little
incremental cost has been added  to  the PCB  in  the  form of
expensive components, and  insufficient or excess solder can
be rectified at this point by wiping the board clean and past-
ing again.
Pre-reflow AOI inspects the components prior to solder-
ing. Only presence/absence  inspection can take place at this
point  because  the  solder  joints  have  not  been  formed  yet.
Again,  adjustments  can be made with minimal  cost,  as  the
component can be simply removed and replaced.
Post-reflow  AOI  can  inspect  both  the  component  and
the  joints  because  the  solder  has  set  at  this  point. Hidden
joints cannot be  inspected with AOI because  this  technique
relies on  visible  line of  sight  to  the  joint. Components  can
be repaired  if defects are  found,  though  the repair cost will
be higher now because  the  existing  component needs  to be
removed, replaced and resoldered to the board.
AXI  can  be  used  to  detect  component  and  pin-level
issues,  as  well  as  find  defects  within  the  solder  joints  or
hidden defects under  components due  to  its ability  to  look
through  the  board  or  component. The AXI machine  takes
various images of the PCB at different focal lengths to build
up a composite image sectioned through the PCB.

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Test Equipment
There  are  four  categories  of  electrical  test  equipment:  In-
Circuit Test  (ICT), Boundary Scan Test  (BST), Flying Probe
Test (FPT) and Functional Test (FT).
ICT  uses  electrical  test  techniques  to  isolate  each  com-
ponent  on  the  board,  confirm  that  the  connectivity  of  the
design is correct and deduce that the board should, in theory,
operate correctly. In reality, certain circuit configurations and
functional defects limit the detection capability that, in turn,
reduces  the  overall  fault  coverage  achieved with  this  tech-
nique.  ICT uses a custom  fixture  to convert  the  tester  inter-
face through a bed of nails to the PCB. These fixtures can be
expensive to produce, so test points should not move during
PCB revisions. There are different diameter-sized test probes
that are used, and  the  smaller  they are,  the more expensive
and less reliable they are. Placing test points too close to each
other may result in a costly smaller probe being used. Spacing
them out  reduces  the  concentrated areas of pressure on  the
board, as well as the fixture cost.
The BST  test  is based on  the  IEEE 1149  standard. The
BST  test performs manufacturing defect detection  via  addi-
tional circuitry that is built into certain types of components
to  allow  access  to  the  complex  component  circuitry.  This
cannot  be  realized  if  the  design  has  not  accounted  for  the
inclusion of  the Boundary  Scan  test.  So  if  your design uses
Boundary  Scan  parts,  ensure  that  the  scan  chain  has  been
implemented correctly.
FPT uses similar techniques to ICT, but instead of using a
custom fixture interface, it uses a handful of movable probes
that contact  the PCB. Due  to  the physical movement of  the
probes,  the  test  times  associated  with  FPT  will  be  signifi-
cantly longer than ICT, but for small batches and quick turn
around, it can be used as an ICT fixture if not required.
FT covers a wide number of specific test systems that are
primarily used  to confirm  the operation of  the PCB  instead
of  for manufacturing defect detection. The  test  systems  are
usually customized for a specific application.

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Conclusion
As  can  be  seen  from  the  description  of  the  typical  PCB
manufacturing processes, you  can  see  that a  lot happens  to
the PCB design  to realize  the actual product  that ultimately
generates a return on the investment for the company. It also
demonstrates  how many  decisions made  during  the  design
phase need  to address  the ability  to design a working PCB,
as well as the ability to manufacture that PCB in an efficient
manner. Design tools that address the needs of not only PCB
designers,  but  also  PCB  assemblers  help  these  two  groups
collaborate  more  effectively  to  deliver  current  and  future
products to market in a timely manner.