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July 2000

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Subject:
From:
Becerra Alejandro <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 13 Jul 2000 11:19:06 -0500
Content-Type:
text/plain
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text/plain (170 lines)
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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