Al,
Here's a couple of links for aperture sizes for adhesive printing
from Alpha and Heraeus.
http://www.alphametals.com/products/techarticles/soldrpstncl/guidelines.html
http://www.4smt.com/smt/hcmd_smt1.htm
Adhesive aperture design is not a cut and dried process. Remember
that these are starting points based on their adhesives, which may not be
what you're using. Different manufacturers and formulations will react
differently with a given stencil, possibly causing excessive spread or
peaking that can contribute to pad contamination.
To reiterate my previous post, consult the IPC specs to determine if
your current problem is acceptable from a workmanship standard. If all you
are seeing is a small amount on the bottom of the termination that is not
preventing formation of a fillet from the end termination to the pad, you
should be OK. Depending on which revision of -610 you are using, you can
have an acceptable joint with up to either 50% or 75% fillet formation. Do
you want to build all your product this way? No, but it will give you
breathing room while you're optimizing your aperture designs.
One of the problems with printing adhesive is trying to do both
small chip components and higher profile parts (such as SOICs) with the same
stencil. The high profile parts usually require a thick stencil to provide a
high enough dot profile to contact the part and provide a good bond. This
creates dot heights for the chip components that are too high, and they peak
and fall or spread to the pads when parts are placed. If this is part of
your problem, you may want to look into the Loctite VariDot stencil
technology. It is a flexible plastic film stencil that is used with acrylic
blades. By defining the snapoff height, it can create dot heights for
components from 0402 to SOICs all in the same print stroke. I have used it
successfully in previous lives, it's really quite cool to see it in action.
Loctite will evaluate your requirements and tailor the apertures for you
based on their knowledge of how the stencil responds. The only drawback is
that they are fabricated at the Loctite facility in Ireland, so your turn
will run week to two weeks. I had heard that they were considering
discontinuing sales to new customers because of disatisfaction with
leadtimes, but that may have changed. Call your Loctite rep and see what
they're doing with it these days. The links below will give you a good idea
of what it is and how it works. Times like this I'm glad I bookmark good web
pages for later reference....
http://www.chemical.felpro.com/electronics/tp_multipoint.html
http://www.loctite-europe.com/wwdh/us/i183ch11.htm
http://www.loctite-europe.com/eu/PDF/Brochures/asei-VARIDOT.pdf
Mike McMonagle
Senior SMT Engineer
Telxon Corporation
(713) 307-2443 Phone
(713) 307-2581 Fax
www.telxon.com
' Innovative Solutions for
Mobile Information and
Wireless Communications'
-----Original Message-----
From: Becerra Alejandro [mailto:[log in to unmask]]
Sent: Thursday, July 13, 2000 11:19 AM
To: [log in to unmask]
Subject: Re: [TN] Adhesive over pads.
Gary,
Thanks for your comments.
I have a question about the data of the aperture sizes:
Is there a reference where I can get this data or you obtained this data?
My question is due to we are looking for some "ideal" aperture size,
actually we are
testing the aperture size recommended by the stencil vendor.
I cross-sectioned some components and I found that there are some components
that don't have solder under their terminals. They only have solder at the
end of the terminal.
Is it acceptable this condition?
Regards,
Alejandro Becerra
-----Original Message-----
From: Smith, Gary [mailto:[log in to unmask]]
Sent: Thursday, July 13, 2000 9:05 AM
To: 'TechNet E-Mail Forum.'; Becerra Alejandro
Subject: RE: [TN] Adhesive over pads.
Al,
excerpts from my research paper (currently STILL in progress). Since I'm
only an intern I can't say this info is accurate due to the project not
being completed:
Adhesive dot profile (size) is also controlled by the internal
diameter/standoff height ratios of the selected nozzle. Typically, dot
height-to-width ratios range from 1:5 to 1:1.5 (h/w = 0.20 - 0.67) mils,
depending on the dispensing system and adhesive grade. The dot size and
volume of the adhesive are more critical factors than the dot shape.
The dot must have sufficient height after dispensation to be greater than
the combined thickness of the pad/land on the PCB and the thickness of the
SMT component's metallization (solder contact area). After the attachment
of the SMT component to the PCB substrate, a gap must exist between the edge
of the adhesive dot and the PCB pads. The illustrations on the following
page assist in clarification of these principles. 1
Industry standards have established no minimum gap between the edge of the
adhesive dot and the edge of the pad. The essential requirement is that the
adhesive does not get on the pad. It can get almost to the edge and may
even touch it but it should not get on the pad. In a case of adhesive
touching the pads of the substrate, an excessive volume of adhesive was
dispensed.
The distance between the bottom of the component and the top of the PC board
is typically between 0.003" to 0.005" (3-5 mils). Another good rule of
thumb is to utilize a dot height that is twice the distance between the
bottom of the component and the top of the PC board. This will assure a
good bond or wetting of the adhesive between the board's surface and the
bottom of the component during placement onto the PC board.
Quantification of adhesive dot dispense volume will depend on the method
selected. If a stencil process is utilized the amount dispensed is
dependent upon the thickness of the stencil and the diameter of the
aperture. The total dispense volume per dot is calculated by utilizing the
standard geometric formula for the volume of a cylinder:
V = P (D/2)2 h
· V - total volume of adhesive dispensed per dot
· D - diameter of the stencil aperture
· h - Thickness (Height) of the stencil
NOTE: Since stencil adhesive printing allows for other shapes besides
circular ones to be printed the volume is dependent upon the stencil
thickness times the area of the aperture opening.
Listed in the table below are approximate recommended aperture sizes for
different components when printing with a metal, laser-cut stencil of 10-mil
thickness using the printing parameters described.
Component Size Single Dot Approach
(stencil opening
diameter in mils) Typical Multiple Dot Pitch
(pitch in mils) Multiple Dot Approach
** (stencil opening
diameter in mils)
0402 12 - 16 N/A N/A
0603 16 - 20 15 2 X 20
0805 20 - 24 20 2 X 24
1206 40 - 47 24 2 X 32
Mini MELF 40 N/A N/A
SOT 23 40 28 2 X 28
1812 50 - 60 40 2 X 55
SO 8 N/A 43 3 X 55
SO 14 N/A 43 3 X 55
** For the Multiple Dot Approach for chip (leadless) components, position
the dots so that between 0.5 and 0.3 of the dot area is under the body of
the component. The Multiple Dot Approach is very effective for preventing
skewing of chip components, especially MELF (Metal Electrode Leadless Face)
types.
-----Original Message-----
From: Becerra Alejandro [mailto:[log in to unmask]]
Sent: Thursday, July 13, 2000 10:25 AM
To: [log in to unmask]
Subject: [TN] Adhesive over pads.
Hi to all,
We are testing stencil-printing process for adhesive dispensing in bottom
side for 0603 components.
We found that there are some adhesive dots that are touching the pads.
We found that the maximum distance that the dot penetrates the pad is
approximately 0.0025".
Is there any standard that defines if it is acceptable to have a small
quantity of adhesive over the pads?
Is there any thumb rule that indicates the size of the stencil aperture.
We have a 0.028" clearance between the pads, and an average dot size of
0.0225".
Thanks,
Alejandro Becerra
Phone (915) 841-8518, Fax (915) 841-8518
[log in to unmask]
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