Daniel - see below, and good luck with the report.
Peter
Daniel Themann <[log in to unmask]> 15/08/2002 06:30 AM
Sent by: TechNet <[log in to unmask]>
Please respond to "TechNet E-Mail Forum."; Please respond to themandj
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cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
Subject: [TN] Smoke Detector Circuit Board
Dear List,
I recently contacted the Australian Surface Mount and Circuit Board
Association
with some technical questions relating to PCB manufacturing, but their
instructors are currently traveling and Dianne Hunt recommended that I
contact
Technet. I am a student at the University of Cincinnati and my teammate
and I
must write a report describing in detail how a circuit board is
manufactured.
I recognize that you are certainly not employed to assist college students
with
technical questions, but if you have a moment and could address the
following
points without too much trouble, I would very much appreciate your help.
Our circuit board comes from a smoke detector and is approximately 1.5 $B!I (B by
3.5 $B!I (B. It contains both surface mount (resistors, capacitors, etc.) and
through-
hole components (ionization chamber, electric horn, battery contacts,
etc.).
These components are placed on ONE SIDE ONLY. We have tentatively drawn up
the
following process steps.
1. Screen Print Solder Paste on side one
**OK**
2. Place surface mount components on side one
**OK**
3. Dry paste
**No! - drying the paste will remove the volatiles from the flux and
wreck the wetting action. Boards should be reflow soldered as soon
after component placement as possible (without drying)to minimise
solderability problems.**
4. Reflow Solder
**OK, but if using water-soluble flux, boards should be cleaned here
to remove residues ASAP before they harden. You would probably be
using no-clean, though, on a smoke detector board**
5. Insert through hole components on side one
**OK**
6. Invert Board and apply adhesive on side two
**What purpose does adhesive serve when, from your description, there
is nothing on side two?**
7. Wave solder
**OK. You might want to consider clinching or half-clinching two
diametrically opposing leads beforehand to prevent components rising
out of the board on the wave. There are other component-holding
techniques as well to prevent this happening.
**Less likely on something like a smoke detector board, mask any
empty component holes on the solder side of the board (side 2) where
there are component shortages or modification options. This prevents
the wave filling them with solder, if you want to use these holes
later. For something as commercial as smoke detector boards, you
wouldn't build with component shortages, or modify them once built,
as this is more trouble and cost than it's worth, and masking is an
additional operation that's also additional cost and time.**
8. Final Clean
**OK**
**9. De-panelise.**
Question 1. Does the process described above seem reasonable? In
particular,
is step 6 necessary if all components are applied to one side only?
**See above**
Question 2. What type of batch sizes would be reasonable in $B!H (Blow
volume $B!I (B and
in $B!H (Bhigh volume $B!I (B processes for a board of this type and size?
**I'm in the Aerospace industry where all our products are considered
"low volume" - typically anywhere between 1 or 2 for prototypes and
50 units for entire production runs. "High volume" depends on your
line set-up, whether you work shifts or around the clock, and how
many component sets your machines can handle before they need to be
re-loaded - in other words, the resources you have at your disposal
to do the job and if they're shared with other jobs or not. Also the
solder paste stencil will need to be cleaned every few prints (could
be automatic), and fresh paste added (could also be automatic).
Dream-up the sort of assembly line that could make this board, add in
any limitations you like (lack of people, the line is needed two days
a week for another job, the people only work 9-5, etc., then use the
set-up to explain any economic batch size you think of. Typically it
will be a case of how many boards do you want to build, or how many
boards can you process in a working day, given all the process run
times, time for documenting the progress of the job. Bear in mind
that one board panel does not have to be complete before the next one
starts. **
Question 3. What would be approximate setup and run times for each
process? We
need these times both for our process route sheets and as input into
our cost
analysis.
** Aerospace product processes tend to be slower than high-volume
commercial stuff because there isn't the same commercial urgency or
to deliver lots of product to an eager population of consumers. More
care has to be taken, and the High-volume machinery is more expensive
in general than low volume stuff, which is therefore less efficient
(i.e. slower). So use my answers as a guide only and hope that
someone in the mass production arena can give you more accurate info.
**
**Solder paste printing - maybe half an hour to fit the stencil to
the printer, do a test print and inspect it for height, coverage and
accuracy. Run time per panel of boards (they're probably panelised
until removed after the last operation, rather than being individual
boards)is maybe 1 to 2 minutes.
**P&P - If this is a first ever run, a Pick & Place programme will
have to be created from the Neutral file information in the Gerber
data (Gerber data digitally describes every feature the PCB,
including the component layouts, sizes orientations and location on
the board). For a small, simple, board, this might take an hour to do
and check. Component loading and checking to the P&P machine - for
such a small board, maybe 1-1.5 hours to fit components to feeders,
load feeders and check loading is correct and load up the placement
programme. Add another half hour or so if you want do a confirmatory
run and inspect it for alignment, placement and so on (called an FAI
(First Article Inspection) run where a bare board is covered with
sticky plactic sheet (sticky side outwards) and the components
populated onto the adhesive. This is then inspected manually or using
AOI machine for correct placement/non-placement, orientation,
alignment and so on). Population of a panel of boards with SMT parts
is probably 1 to 2 minutes**
**Wave solder - fitting boards to conveyor frame, setting up the
conveyor speed, setting pre-heater temperature, wave height and so
on, is maybe half an hour if parameters are known. Maybe half a day
to establish all the parameters if the board run is the first ever of
its type. Cycle time is about 5-7 mins. once the parameters are known
and set.
**Cleaning - fairly standard process with no specific set-up. Cycle
time is about 10 minutes, and if using water wash, maybe half an hour
in an oven to dry the boards afterwards.
Question 4. Is it possible to purchase the board itself (without the
components
attached) from an outside vendor and to simply attach the components
in-house?
Or does the manufacture of the board and the soldering processes by
which the
components are attached constitute one continuous process? If the
board itself
can be purchased outside, how much might it cost given low and high
volumes?
**PCB's are normally fabricated by a specialist company for
population by its customer. There are companies, though, that offer a
"turn-key" service - i.e. they will take charge of having the PCB
made, the components bought and the whole lot assembled and tested
for you. All you have to do is to supply them with the relevant
drawings and parts lists. This is an option if you have no assembly
facilities yourself, and depends on relative economics (i.e. it's
cheaper to subcontract everything rather than invest in the process
machinery yourself). In short, fabrication houses specialise in
producing bare boards and assembly shops specialise in mounting
components to the bare boards. Usually, these specialisations are
separate trading entities.**
Question 5. Does you know of any companies that manufacture special
through-
hole components such as the electric horn and the ionization chamber?
Do you
know where prices on such components, and more common components such
as
resistors and capacitors, could be obtained?
**No, but for normal components, you can find any number of component
distributor on the Net. You can probably find your Horn and Chamber
people there too, if you put in the right key search words.**
If you could spare us a few minutes of your time we would be most
grateful. If
you feel that you can assist with some questions, but not with others
we would
still appreciate any insights you can offer. If you know of anyone
else who
might be able to help, please feel free to forward this message on.
The
project is due in less than two weeks so timely responses would be
fantastic.
Thank you in advance for your help.
Sincerely,
Daniel Themann
Arthur Jackson
**Short of actually writing the report for you, I'll try to help with
any other questions you may have (time permitting), unless you
acquire a guru with more relevant experience to help you. You've just
put your toe into a very large, very deep and very murky pond, though
- such is the world of electronics manufacturing.
Good luck. If you get an A*, you'll owe us a beer!
Peter
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