RE: [TN] FPBGA Area Ratio QuestionHi Alejandro,
Thanks for the reply. In your calculation it looks like you used the area of
the stencil aperture as the numerator. However, the formula in the IPC
document uses the area of the pad (land) as the numerator. That is how I
came up with my numbers. For example:
1. Square aperture of .013" with .005" thick stencil plate: Area of .014"
diameter pad = .000154
Area of walls =
.00026
Area ratio = .000154/.00026 = .592
2. Square aperture of .012" with .005" thick stencil plate: Area of .014"
diameter pad = .000154
Area of walls = .00024
Area ratio = .000154/.00024 = .642
Should the formula really use the aperture area instead of the pad area?
Thanks.
Bob
=======================
Robert Barr
Manufacturing Engineering
Formation, Inc.
Voice: 856-234-5020 x3035
Fax: 856-234-6679
-----Original Message-----
From: Becerra Alejandro [mailto:[log in to unmask]]
Sent: Wednesday, May 29, 2002 1:41 PM
To: 'TechNet E-Mail Forum.'; 'bbarr'
Subject: RE: [TN] FPBGA Area Ratio Question
Bob,
I calculate the area ratio using a 5 mil stencil and my numbers do not
agree with yours.
1. Square aperture of 0.013". Area of Aperture = 0.000169.
Area of Lateral Walls = 0.013 X
4 X 0.005 = 0.00026.
Area Ratio = 0.000169/0.00026 =
0.65.
2. Square aperture of 0.012". Area of Aperture = 0.000144.
Area of Lateral Walls = 0.012 X
4 X 0.005 = 0.00024.
Area Ratio = 0.000144/0.00024 =
0.60.
If you take into account the radiused corners the area ratio may be lower.
I calculate for the option 1 (with radiused corner of 0.003") and it was
0.68.
When you reduce the size of the apertures with the same stencil thickness
the area ratio is reduced (not increased)
Regards,
Alejandro Becerra
-----Original Message-----
From: bbarr [mailto:[log in to unmask]]
Sent: Wednesday, May 29, 2002 9:40 AM
To: [log in to unmask]
Subject: [TN] FPBGA Area Ratio Question
On a fine pitch BGA (.8mm pitch, 384 balls, .5mm ball diameter) I am using
a
.014" diameter land per TI's recommendation. I was thinking then of a
.013"
square stencil aperture with .003" radiused corners on a 5 mil plate. This
seemed reasonable until I did an area ratio calculation per IPC-7525. The
result was .592, which fails compared to the acceptable value of >.66. The
aspect ratio is a solid 2.6, although IPC says I should not consider
aspect
ratio in this case, only area ratio. If I reduce down to a .012" square
aperture I can get to an area ratio of .642. Still fails, but close. But,
reducing the aperture 2 mils compared to the pad seems extreme and
violates
the recommendation in para. 3.2.2.4. So my question is, what do you guys
and
gals use for an aperture design on a component like this? Do you go by the
IPC guidelines?
Thanks.
Bob
=======================
Robert Barr
Manufacturing Engineering
Formation, Inc.
Voice: 856-234-5020 x3035
Fax: 856-234-6679
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