Hi Roger, you seem to know Moore than most people about bonding on Ni.
Funny, I recommended Bob also, however, Bob will retire and is dumping
knowledge to younger members, as he says. Another guy who is worth a
call or two is Andrew Niblett at duPont/Bristol. Skilled, adequate, good
advisor.
We are testing 100um Al wire on Au/Ni/Cu FR-4 boards after 1,000hs in
+125, after tempcycling and similar conditions. Until now, no severe
problems, but as you mention, one has to work intimately together with
the board manufacturer and always with one eye open. We have built a new
automatic line with a wedge bonder as one component, so we have to
believe in Al bonding on Ni.
I may drop some more information in your mailbox later (we cooperate
partly with ViaSystems).
See you....
Ingemar Hernefjord
Ericsson Microwave Systems
-----------------------------sep-------------------------------------
> Wolfgang,
>
> As Albert says, many of the problems associated with
wire
> bonding arise from the quality of the materials, set up of the
> machines, the design of the product and the bonding jigs being
used,
> with out knowing much about the returns you are getting (are they
not
> bonding or just having low strength etc?) here are a bunch of
> comments.
>
> PCB pads,
> Need to be elementally "clean" on the surface, most
> contaminants that lie ontop of a plating will either act as a
> lubricant which will reduce bonding energy, but more importantly
will
> also run the risk of being held within the bond which can
precipitate
> voids and lead to early failure. Auger electron microscopy, is
great
> for this analysis, but expect to see loads of C and O anyway
> Pad metals themselves need to be "pure" there are
loads of
> papers on this requirement, and reading between the lines they
suggest
> that the total impurities within a plated layer should not exceed
1%,
> with each individual impurity not to exceed 0.1%, this is a
massive
> level of junk in a plating, and most good plating houses and
board
> suppliers should be able to maintain levels well below this. One
word
> of warning, dont allow H or Tl to get into the plated layers,
> especially into Au, as they can lead to embrittled layers, and
> outgassing during bonding or subsequent heat treatments.
> Bonding metallurgy, care should be taken when plating
a
> single board that will have both Au and Al wires bonded to it,
while
> Au bonding likes nice soft thick Au, Al should only have enough
Au to
> protect the underlying bondable layer, this requirement for Al is
due
> to the famous Kirkendall voiding (previously known as purple
plague)
> which is related to the diffusion couples that operate between Au
and
> Al.
> Total pad hardness, ideally this should be matched as
close
> as possible to the hardness of the wire material being used, if
one is
> harder, it will scrub away at the other and give a poor bond.
>
> Machine set up,
> This is probably out of your hands, but a well run DOE
can
> often take a lot of ambiguity out of the bonding process.
> Make use of both the pull and shear tests. Pull will
let
> you know where the weakest point of the bond is, it will probably
be
> the heel of the bonds in Au, but the wire itself for the Al. If
the
> feet lift, there is a major problem, either with the bond
parameters
> or the plating quality. Shear testing lets you measure the actual
> strength of the layers holding the bond together, and the failure
> point should be in the foot of the wire itself, and never within
the
> pad or intermetallic.
>
>
> Design.
> Substrates are often not fully supported across their
area,
> this can lead to ulatrasonic attenuation and energy loss during
wire
> bonding. basically the pads vibrate, and the ulatrasonics cant do
> their job, check that the pads are supported.
>
> Wire geometry, and type
> Wire shape greatly effects the reliability of bonds,
too
> long, too short, too tight, too high, its a mine field, but most
> decent manufacturers of modules know what they are doing.
> Wire materials vary, the Au will be doped, but does it
have
> a high elongation? if not, it may be work hardening too much
during
> step back and embrittling the wire. for Al, at about 10mil,
there are
> debates as to what is best 99.99 (4N) or 99.999 (5N) purity. I
would
> stick with 5N at this size, its more ductile, which gives better
> flexibility during bonding, and it can support its own weight
during
> vibration etc.
>
> Failure
> There are really two known problems with wire bonds
> 1) they dont stick when bonded
> - check pull and shear strength
> Depending on point of failure review parameters
> - listen during bonding, if there is a high spitch
buzz,
> there is vibration in the system somewhere, "seek
and
> destroy"
> - getting the plating analysed, and look for high
surface
> contaminants
>
> 2) They fall off, or break during service
> - Check the Metallurgy, are they compatible? not Al
onto
> thick Au
> - Check pull and shear strength when bonded, and after
> various heat treatments (identify what switches it
on and
> off and examine the joints closely
> - If bond quality is good, and then drops off rapidly,
then
> its probably something to do with contamination
within
> the system (check the plating etc)
> - Its not Kirkendall voiding is it?
> - Are the wires breaking in the span? check wire
geometry
> wire type, bond quality, overbonding?
>
> On the hole, FR4 should be easily bondable with both Au and Al
> wires, providing that the pads are clean, the metalllurgy is
> compatible, and the bonding process is under control.
>
> I hope this helps in some way, if not read through Harmans,
famous
> book on bonding "Wire Bonding in Microelectronics" its got
everything
> covered, and should be the bible for most Bonding Engineers
anyway.
> Another good contact is Bob Clements at TWI in Cambridge, he
knows his
> stuff, and has an equiped lab to do some trials with.
>
>
> Good luck
> Roger
>
>
> Roger Massey
> Materials Technologist
> Motorola AIEG
> Stotfold
> England
>
>
>
>
> ______________________________ Reply Separator
_________________________________
>
> Subject: Re: [TN] wirebonding
> Author: "TechNet E-Mail Forum." <[log in to unmask]> at #email
> Date: 13/07/99 09:02
>
>
> Dear Mr. Wolfgang,
>
> I think thatyou have to consider the parameters from the bonder and
the
> quality of PCB itself/ the die.
>
> For the bonder parameter, you know exactly the time, pressure for the
right
> bonding performance and of course, the wear and tear of the aluminum
wire
> pointer will affect the bondability as well.
>
> For the PCB, you have to specify to your board maker they must supply
to
> you boards which are ultrasonic bondable. The important factors for
good
> bondability PCBs are nickel thickness, nickel hardness, the gold layer
and
> the cleanliness of the surface going to be wire bonded. For the die,
you
> usually do not have to acknowledge since they are ready for bonding.
>
> In my experience, poor bonding come from the PCB quality and the
improper
> adjustment of bonder.
>
> Hope this help.
>
>
________________________________________________________________________
____
> _____________
>
>
> At 03:10 PM 1999/7/12 -0400, Erat, Wolfgang wrote:
> >Good Day
> >
> >Would appreciate any and all input on reliability data / experience /
> >potential failuremodes when wirebonding Alu wire to electroless
nickel /
> >immersion Gold boards.
> >
> >HiRel Automotive application
> >wedgebonding
> >ultrasonic wire bonding
> >10 mil AL wire
> >Au 3 to 5 microns
> >
> >appreciate your response
> >
> >[log in to unmask]
> >
> >
> Albert Mok
>
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