Dear Roger,

Thank you for the comprehensive and fruitful information.

____________________________________________________________________________
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At 09:45 AM 1999/7/13 +0100, Roger Massey-G14195 wrote:
>     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
>
>
Albert Mok