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TechNet E-Mail Forum <[log in to unmask]>, Inge <[log in to unmask]>
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Mon, 19 Mar 2007 18:59:34 +0100
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 Our subcontractor would like that gold! What's the price per kilograms?
What are the delivery terms? Yum-yum-yum..
Inge



----- Original Message ----- 
From: "Creswick, Steven" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Monday, March 19, 2007 6:02 PM
Subject: Re: [TN] Gold Ball Bond attaching surface roughness


Victor,

If I found the correct pictures on Steve's web site - Ball bond 1, 2,
and 3, [upper right side of page 1] you do have a somewhat rough surface
to bond to.

You are unlikely to find someone that can give you a specific surface
finish requirement which is equivalent to a given degree of reliability.
Just as Ingemar says, there are many variables coming in to play - bond
schedule, plating quality/hardness, cleanliness, clamping, tool style,
etc.

Very rough surfaces can be bonded quite well - aka some thick film
[cermet] golds [spoiled by in-house golds which were essentially
glass-free at the surface, whereas many others required burnishing].  I
believe this to be because of their 'soft' nature.  Whereas, a
mirror-smooth, plated cobalt-doped, edge connector gold is virtually
impossible to gold ball bond to simply because it is so very hard.  The
point being made is that just looking at surface finish without taking
into account the plating is meaningless.

Your substrate, the copper, is basically 'hard' when compared to the
pure gold ball.  If your bond schedule is on the lite-side [and the
plating is bondable], you will bond to the peaks only.  You may get
decent pulls, but you will generally not fully involve the full
underside of the ball bond.

To an extent, you can increase your bond energy by either cranking up
the temp, time, and/or power and get more deformation of the ball bond.
You will likely involve more than just the peaks, but to what extent can
only be judged by doing wire pulls and accelerated aging.

Then you can do what some folks do and crank up the bond force and smash
the bejeebers out of the bond without necessarily improving the
reliability of the bond - but it certainly is smashed into the
substrate!

The more energy I toss in at this point, the greater the opportunity for
weakening the bond in the region directly above the ball. This is what I
would call a first bond neck/heel break.  There is a heat affected zone
in the wire immediately above the freshly formed free air ball, as a
consequence of the flame-off step of the bond sequence.  Just because I
get a neck break at this point during wire pull [meaning the ball stuck]
does not give me the warm fuzzies.  It just means the neck was weaker
than the ball bond ... what did it fail at magnitude wise??

Plating can really play a big roll in apparent surface finish.  From the
pictures, it does not look like gold.  Are you bonding to silver?  Did
you say what your plating scheme was??  A nice nickel barrier plate
followed by a [soft] silver plate should hide many of the asperities of
the underlying copper ....

As Ingemar says, you could always go in and coin the bottoms of your
divots before [or after] plating to help improve the surface finish.

Alternatively, if you have equipment which can put down a gold bump into
the divot, then you could place your ball up on the chip and the second
bond down on your bump.  I find that getting balls to bond to difficult
surfaces is generally much easier than getting the second bonds to stick
in the same place.

In conclusion, your surface is not as rough as it potentially could be.
A great deal is going to depend on your plating scheme.

I can't give you a generic number to use for surface finish, but common
wisdom is that the rougher the surface, the more likely you are to have
bond failure.

Steve



-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Hernefjord Ingemar
Sent: Monday, March 19, 2007 11:40 AM
To: [log in to unmask]
Subject: Re: [TN] Gold Ball Bond attaching surface roughness

The matters you speek of were handled decades ago.
Try to find any of these :

- J.m. Vandenberg, R.A. Hamm, A continous X-ray study of the interfacial
Reactions in Gold-Aluminum Thinfilm Couples. J.Vac.Sci. Tech 19 (1981)
84-88

- E.Philofsky, Intermetallic Formations in Gold-Aluminium Systems,
Solid-State Electronics 13 (1970) 1391-1399

- G.Majni, C.Nobili, G.Ottaviani, M.Costato, E.Galli, Gold-Aluminium
Interactions and Compound Formations, J.Appl.Physics 52 (1981) 4047-4054

-B. Lan, S. Pinamaneni, Thermosonic Gold-Wire Bonding to
Precious-Metal-Free Copper, Proc. 38th Electronic Components Conf.(IEEE)
L.A. Cal. 1988, 546-551

-J.Falk, J.Hauke, G.Kyska, Wirebonding on PWB, Circuit World 20 (1994)
8-13

-K.Toyozawa, K. Fukita, S.Minamide, T.Maeda, Development of Copper Wire
Bonding application Technology, IEEE Trans.on Components, Hybrids and
Manuf.  Technology CHMT-13 (1990) 667-672

These are what I can give you, I have many more references, but I have
no time to dig just now....maybe later.

If you have good luck, George Harman may read these lines, and give you
exact answer at once.


I have performed studies of the nano/micro-welds that take place after
thermosonic ball bonding, and if we speak in very general terms, the
asperity ought to be in the range 0.1 - 0.5 micrometers, much depending
on the microhardness of the parties. These small bonds take place at
temperatures as high as 2,000 Centigrades during milliseconds, and the
general experience is, that most important of all parameters are these:
cleanliness (lubricant free), matching microhardness, matching
ultrasonics.  The surface roughness may come as number four or five, I
don't know.  Furthermore, if the surface isn't smooth enough, you can
place a special tool on the horn instead of the capillary or wedge, and
"polish" by means of the ultrasonic and the pressure.

As I said, Trikeman is more guru at this, 125 years at CTS must exist
still in his organic computer.


Good Luck

Inge




-----Original Message-----
From: [log in to unmask] [mailto:[log in to unmask]]
Sent: den 19 mars 2007 15:40
To: [log in to unmask]; Hernefjord Ingemar
Subject: RE: [TN] Gold Ball Bond attaching surface roughness

Inge,

   Thanks for sharing the HCC document with me.   Very interesting
article.
My main concern is to find out guidelines for ball bond surface
roughness to ensure adequate IMC formation.

Victor,

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Hernefjord Ingemar
Sent: Monday, March 19, 2007 9:08 AM
To: [log in to unmask]
Subject: Re: [TN] Gold Ball Bond attaching surface roughness

HCC are great too, they really know how to define nailheads etc.
Inge

http://www.hccindustries.com/files/HCC-Package-Design-Guide.pdf



-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Creswick, Steven
Sent: den 19 mars 2007 11:21
To: [log in to unmask]
Subject: Re: [TN] Gold Ball Bond attaching surface roughness

Ingemar,

I agree with your pin oscillation caution statement!

Might I ask if your plater is local or outside, of the country?  Am
assuming Electroless on your pins.  Correct?



Alas, grounded for another weekend.

Fear not.
Will not drop!

Steve

-----Original Message-----
From: Inge [mailto:[log in to unmask]]
Sent: Friday, March 16, 2007 3:02 PM
To: TechNet E-Mail Forum; Creswick, Steven
Subject: Re: [TN] Gold Ball Bond attaching surface roughness

Both, in fact. Yep, the plater cooperates understandingly. Nota bene: if
you bond on nailheads or any similar pins, you need keep an eye on the
U/S frequency. The pins can swing with the horn, and that can cause poor
adhesion.

So, Trikeman, you begin to feel altitude excitement. I understand. I've
seen young bird's whole body vibrate  and suddenly they can't resist any
longer, but jump from the bird nest and swing up in the sky, anxiously
but triumphing. Go, Trikeman, fly with the eagles! But don't drop things
on my head. (I don't know how much bird you become up there)

Inge



----- Original Message -----
From: "Creswick, Steven" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Friday, March 16, 2007 11:12 AM
Subject: Re: [TN] Gold Ball Bond attaching surface roughness


Okay Ingemar,

Are you putting the ball on the [nail head?] pin, or the wedge?  I bet
your plating/plater has a lot to do with your success  :-)

Sometimes you crack me up!


I need the weather to calm down so I can go flying.  Starting to grow
roots have been on the ground so long.  Finally loosing enough snow that
I might be able to get out on the field.

Steve

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Inge
Sent: Thursday, March 15, 2007 5:18 PM
To: [log in to unmask]
Subject: Re: [TN] Gold Ball Bond attaching surface roughness

Copper Cup...when is that?

Sorry, I know you are not to play with until on Friday.

Even if the surface you are looking at may seem rough in a macro-looking

perspective, the micro ditto may serve well for the ball or wedge.
Hence, we
do bonding on rough glass feed-true pins, which are absolutely not
thought for bonding, but it works fine. Try! And do pull test!

Inge


----- Original Message -----
From: <[log in to unmask]>
To: <[log in to unmask]>; <[log in to unmask]>
Sent: Thursday, March 15, 2007 7:48 PM
Subject: RE: [TN] Gold Ball Bond attaching surface roughness


The application I am referring to is a device where the ball bond, BCC,
is placed on a copper cup which also serves as the solder fillet to the
PWB pad.
This cup is really rough exhibiting tooth like structure similar to
those of the vendor treated side on copper foil.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Inge
Sent: Thursday, March 15, 2007 12:23 PM
To: [log in to unmask]
Subject: Re: [TN] Gold Ball Bond attaching surface roughness

Agree with Trikeman, the semi process is a epitaxial process using
"refractory" metal, and the asperity is not measureable on normal
tallysurfers, i.e. <0.1 um.  Only situation may be with semi chips with
plated bond pads, but then we talk gold. And these have never caused us
any
problems either. You needn't bother. /Inge

----- Original Message ----- 
From: "Creswick, Steven" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Thursday, March 15, 2007 1:36 PM
Subject: Re: [TN] Gold Ball Bond attaching surface roughness


Victor,

Sorry, don't have a number to give you.  We do it all day long ... and
the IC mfr's know how to make the bond pads....

Typically smoother is better, but if one were to look at the plating on
a lead frame at high enough magnification, it is certainly not smooth
either.

Possibly someone has some form of spec on the surface finish of a bond
pad.  All that I am aware of is pad thickness.

If no one has a surface finish number, I might be able to run some next
week.

Steve C

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Victor G. Hernandez
Sent: Thursday, March 15, 2007 8:25 AM
To: [log in to unmask]
Subject: [TN] Gold Ball Bond attaching surface roughness

Fellow TechNetters:

Typically I have observed ball bond attached the alunium window of a
chip, and copper lead frame.   Are there any criteria for the roughness
of this surface for establishing a good intermetallic formation?

Victor,


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