Ah, Glen!
I've been battling this one for far too long now myself, and one of the
questions I posed was just the one you are posing here - is it imore
realistic to calculate the solder paste area, since this is where all the
flux contamination wil be concentrated, or go with the spec requirements
which gives a cleanliness requirement calculated from 2x entire board area?
I have my own opinion which I'll explain in a moment, but let's look at
history first. The cleanliness figures in the IPC and MIL specs are now
over 30 years old and date from the days when components were all PTH and
wave soldered. With wave soldering, the entire board area is fluxed, and
with the technology of those days, trace weights, widths and spacings were
much greater than they are today. Hence the cleanliness level required was
not as stringent as it needs to be today. The cleanliness level, taken over
the entire board area made sense then, but has not been updated since to
account for the much finer traces, closer spacing and SMT technology of
today. SMT technology, of course, largely uses Convection Reflow soldering
techniques in which solder paste (flux) is deposited only in localised
areas.
Doug Pauls from Rockwell Colins will probably give you a very knowledgeable
answer to your question, but my opinion is this:
a) if you want your boards to meet IPC or MIL spec, it can be easily done
using the current ROSE test equipment, which also has changed very littlle
in principle in 30 years. Take whatever reading you get from the machine at
face value, divide it by the total board area and you have the appropriate
figure against which to judge if you've passed or failed. However ...
b) Industry concensus seems to have it that a much tighter cleanliness
figure is now required than in days of yore - 0.2ug NaCl eq cm^-2 instead
of 1.56ug is the figure that's been bandied about, based, I believe, on SIR
and Ion Chromatography results for modern boards. It's a figure I prefer to
believe than the IPC spec figure.
c) Using the solder pasted area as the divisor for the cleanliness result
makes the test much harder to pass, but will give you (to my mind) a more
realistic idea of how clean the fluxed areas are. I think, too, that the
real truth about overall board cleanliness will lie somewhere in between
the two areas, though where I cannot say for sure but probably nearer to
the pasted area than the overall board area.
d) as boards get smaller and clealiness level requirements become tighter,
we run into problems with the ROSE testers, which are still happily
claiming compliance with the IPC and MIL Specs and tend to assume a board
area that may be much larger than you normally deal with. The problems are
those of working at the extreme limits of the machines' capabilities. Sure,
you can continue to do what you do now, which is to test a number of boards
at the same time to, as you say, give the machine something to measure, but
I believe it's time that the ROSE tester manufacturers looked to updating
their technology.
I've been giving Mike Konrad at Aqueous Technologies a hard time, demanding
"official" figures for how well their Zero Ion reads at low levels of
contamination, because from tests I have carried out, machines tend to
start reading inaccurately when total contamination weights drop much below
1000ug NaCl eq. I have boards that measure just over 9" x 6" and have a
target cleanliness figure of 0.2ug NaCl eq cm^-2. This means that the
maximum permissible contamination weight must not exceed 143ug per board.
The ROSE test tank I have is large enough to allow two boards to be tested
at one time, which means that the machine has to be capable of reading
accurately 286ug NaCl eq (board area is irrelevant as it's just a divisor
that you programme into the machine yourself - it doesn't affect the
machines ability (or otherwise) to detect and measure the contamination
level. 286ug is well below the level at which the machines seem to read
accurately, or consistently. The variance at these levels is also great,
and if my tests are accurate, makes the measurement at these concentrations
meaningless.
As more people move to "no-clean" technology, the only folks left using
ROSE testers will be the Class 3 board builders who continue to use
water-soluble fluxes. Maybe that market is not big enough to justify
re-evaluating the capabilities of the testers, in which case ways round the
capability problem have to be found.
Glen, whether you choose to use overall board area or pasted area for your
cleanliness calculation is up to you. If you want a better idea about the
board cleanliness, use pasted area, but if you want the boards to pass test
for shipment, use overall area adn no-one can legitimately criticise you.
Hope some of the other gurus will give me a debate on ROSE tester
capability.
Peter.
Glenn Pelkey <[log in to unmask]> 23/05/2003 09:59 PM
Sent by: TechNet <[log in to unmask]>
Please respond to "TechNet E-Mail Forum."; Please respond to glenn.r.pelkey
To: [log in to unmask]
cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
Subject: [TN] ROSE for Flip Chip
Hi Technetters,
My buddy and I have a disagreement on using Resistivity of Solvent
Extract (ROSE) for measuring cleanliness on a Flip Chip board. So, we're
throwing it to league of experts on this forum for feedback
Here's the story: The board and assembly are very simple. One
flip chip die at about 0.002 square inches and one board about 0.490 square
inches. Flux is applied only to the flip chip die, reflowed, and DI inline
washed.
Here's the disagreement: I say use 2X the surface area of the die
for the calculation. My good friend says use 2X the board surface. Of
course, you can see one number is much higher than the other in ug/sq inch
NaCl equiv. My logic for using only the die area is to understand the
process cleanliness at that location only, but I recognize the entire board
surface could be cross contaminated in some manner. We also loaded the
machine with 32 of these units to get something measurable. When we only
put 4 in, nothing showed up, but the smallest entry for surface area is 0.1
square inches.
Thanks for your input!
Glenn
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