ED-7306_E - 第15页
JEITA ED-7306 - 13 - 3.4 Solder ball bridges after BGA board level assembly The occurrence of the solder ball bridges depends on how much package warps during reflo w process. The mechanisms of the solde r ball bridges a…
JEITA ED-7306
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3.3 Open solder joints after BGA board level assembly
Take BGA as an example, since it indicates larger warpage at elevated temperature. The behaviors of the
package warpage and the solder paste during the reflow process are described as follows:
(1) It is premised that the package is flat with acceptable coplanarity at room temperature and PWB is
ideally flat in all conditions.
(2) The package warps larger in association with rising temperature. Just below melting point, some
crowns of the solder balls may even separate from the surface of the solder paste at the package
corners, where the warpage is larger.
(3) As temperature rises further and exceeds the melting point of solder, the solder balls and paste melt
and collapse.
(4) Even if the crowns of some solder balls were apart from the solder paste just below the melting point,
the collapse of the balls produces good solder connection with the activated solder paste.
(5) Acceptable solder joints are formed after reflow.
(6) In case package warps more than the criteria, the crowns of these balls does not touch the paste
when the balls collapse. It causes the open solder joints.
Under the consideration of mechanisms from (1) to (6), if the sum of package warpage at elevated
temperature and the lowest standoff height is smaller than the sum of the original solder-ball height and the
thickness of the molten solder paste, good solder connection can be expected after the board assembly,
and vice versa. (See Explanatory Fig. 1)
The maximum relative displacement is defined as the difference between the highest and
the lowest solder joint heights of BGA package mounted on the ideally flat seating plane,
where none of solder joints are open.
Explanatory Fig. 1 Calculation of the maximum relative displacement immune
from open solder joints
Highest joint
height of BGA
without open
solder joints
Lowest joint
height of BGA
without open
solder joints
Original
solder ball
height
+
Molten
solder height
Original
solder ball
height
+
Molten
solder height
Nominal solder joint
height of ideally flat
package
Empirical
data
↓
Nominal
joint height
x 0.87
Empirical
data
↓
Nominal
joint height
x 0.87
Ideally flat seating plane
JEITA ED-7306
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3.4 Solder ball bridges after BGA board level assembly
The occurrence of the solder ball bridges depends on how much package warps during reflow process. The
mechanisms of the solder ball bridges are described below:
(1) If the package warpage is less than the maximum permissible warpage just above the melting point,
all solder balls are once soldered to the lands on PWB.
(2) Further elevation of the temperature makes some balls flattened while others stretched because of
the increase in package warpage.
(3) The collapsed balls have larger diameters, while the stretched balls become thinner but are still
connecting the package and PWB owing to surface tension.
(4) When the diameters of the collapsed balls expand beyond the certain percentage of the ball pitch
(80 % of the ball pitch obtained from the experimental data), the failure rate of the short circuits
increases.
Therefore, the maximum relative displacement of the package without the solder bridge is the difference
between the height of the stretched balls (the highest joint height) and that of the flattened balls (the lowest
joint height) whose diameter is 80 % of the ball pitch. (See Explanatory Fig. 2)
The maximum relative displacement is defined as the difference between the highest and
the lowest joint heights of BGA package mounted on the ideally f lat seating plane, where
none of solder joints bridge.
NOTE: Constants of the calculations are obtained from the experiment and used for simplicity.
Explanatory Fig. 2 Calculation of the maximum relative displacement immune
from solder ball bridges
Highest ball
height of BGA
without solder
bridge.
Lowest ball
height of BGA
without solder
bridge.
Empirical
data
↓
Nominal joint
height x 1.3
Empirical
data
↓
Nominal joint
height x 1.3
Nominal joint height of
ideally flat package
Ball pitch>Ball
width
↓
Empirical data:
Pitch x 0.8
↓
Height
calculation
Ball pitch>Ball
width
↓
Empirical data:
Pitch x 0.8
↓
Height
calculation
Ideally flat
seating plane
JEITA ED-7306
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3.5 Maximum permissible package warpage of BGA and FBGA
3.5.1 The maximum permissible package warpage of BGA and FBGA is described in Explanatory Table 1,
which is calculated from the experimental data.
3.5.2 Given that PWB is an ideally flat seating plane, the maximum relative displacement from the seating
plane is the difference between the highest and lowest joint heights of BGA which is immune from the
open solder joints or solder ball bridges.
3.5.3 The maximum permissible package warpage of BGA and FBGA is determined to be 80 % of the
maximum relative displacement, where either open solder joints or solder ball bridges was not seen.
The other 20 % is given to the permissible warpage of PWB. The ratio reflects the difficultness in
maintaining straight of the package versus PWB at elevated temperature, i.e. complexity in the
materials and structure of package vs. PWB.
3.5.4 The criteria of maximum permissible package warpage for solder joints without open or short circuits
are obtained separately. Less than 10 µm of difference indicate that the open solder joints and solder
bridges are the phenomena caused by the same reason but viewed from opposite sides. The current
magnitudes of package warpage barely satisfy the budget allocation of the tolerance, 80 % to the
package. However, along with the progress in technology, the methodology to reduce the package
warpage will be established, and then the criteria will be reviewed.