# SMT STENCIL DESIGN by billkaufman

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```									VOLUME OF PASTE IN THE HOLE
(ALL MEASUREMENTS ARE IN mils) VOLUME OF CYLINDER SUBTRACTING OUT THE VOLUME TAKEN UP BY THE LEAD OF THE COMPONENT 1.333 IS FUDGE FACTOR FOR LOSE OF PASTE IN HOLE FROM COMPONENT INSERTION

VPinH

⎤ ⎡ ⎛ DH 2 − DL 2 ⎞ ⎜ ⎟ × TB ⎥ ×1.333 = ⎢2 × π × ⎜ ⎟ 4 ⎥ ⎢ ⎝ ⎠ ⎦ ⎣

DH = Diameter of hole DL = Diameter of component lead TB = Board Thickness VPinH is in cubic mils (mils3)
STENCIL APERTURE REQUIRED

2 IS COMPENSATION FACTOR FOR LOSS OF VOLUME BECAUSE PASTE IS ONLY 50% METAL BY VOLUME

S AP = (

VPinH ) ST

ST = Thickness of the stencil SAP is in square mils (mils2)
EXAMPLE

DH = 27 mil DL = 18 mil ST = 8 mil TB = 62 mil ⎡ ⎛ 27mil 2 − 18mil 2 VPinH = ⎢2 × 3.1415 × ⎜ ⎜ 4 ⎝ ⎣

⎤ ⎞ ⎟ × 62mil ⎥ × 1.333 = 52,589mil 3 ⎟ ⎠ ⎦

S AP

52,589mil 3 =( ) = 6,574mil 2 8mil

Required square aperture opening is 81 mil x 81 mil. Depending upon pad spacing, vary the aperture so there is 20 mil clearance between holes. Paste in hole can be accommodated well on 100mil pitch parts. RESTRICTIONS MINIMUM CLEARANCE BETWEEN HOLES SHOULD BE 20 mil MINIMUM APERTURE DIMENSION SHOULD BE THE OVERALL SIZE OF THE PAD (hole plus the annular ring).

VOLUME OF PASTE IN FILLET

VF = 0.215 × r 2 × 2 × π × (0.2234 × r + a )

[(

)

]

1 (DH − DL ) + LAR 2 r = Radius of fillet a = ½ DH LAR = Length of the annular ring r=
TOTAL PASTE VOLUME REQUIRED

VT = VPinH + xVF

x = 1 for single sided, 2 for double sided

VT is in cubic inches (in3)

GENERAL STENCIL INFO
PAR (PRINT AREA RATIO)

PAR =

L ×W 2 X (L + W ) × T PAR>0.90 PAR>0.66 PAR>0.50

CHEM-ETCH LASER-ETCH EFAB ASPECT RATIO
AR = W ≥ 1.5 T

VOLUME OF METAL IN PASTE (SN63, SN62) 90% METAL LOAD = 51.7% METAL BY VOLUME 89.5% METAL LOAD = 48% METAL BY VOLUME DENSITY OF FLUX IS ABOUT 1g/cm3 DENSITY OF SOLDER PASTE IS ABOUT 4.83 g/cm3

APERTURE DESIGNS

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