A Study on Package Stacking Process for Package-on-Package (PoP) (cont.)
The package stacking was carried out by using the above-
Screen print
mentioned test vehicle and the dipping material. The dip
thickness was set to be 150um and 250um at the dipping stage
of a multi-functional placement machine. The solder paste of
Sn3.0Ag0.5Cu was screen printed by a 120um thick metal
Bottom CSP
mask on 0.28mm NSMD (Non Solder Mask Defined) pads of
Process #1,2
mount
Process #3
a test board. A conventional reflow profile with a 240C peak
(solder dip)
temperature was applied to the board assembly including
package stacking. After that, 10 units per each condition were
mechanically peeled off, and the number of balls displaying
open joints was counted. Figure 5 is a photograph of an
Top CSP
example observed after the sample was peeled off.
stack
Process #1 Process #2, 3
(flux dip) (solder dip)
Reflow
Process #1 #2 #3
Screen print on board x x x
Dip bottom CSP to Paste
Place bottom CSP x x x
Dip top CSP to Flux Paste Paste
Stack top CSP on bottom x x x
Figure 5: Open solder joints
Reflow x x x
Resultsanddiscussion
Table 4: Experimental process flow
1.Influencebypackagewarpage
The number of open solder joints when the warpage of the
bottom package is changed is shown in Table 6. It was
Experimentalprocedure
observed that only in cases when the bottom package had a
As variables, we studied warpage of the bottom package, large die, meaning it displayed large warpage (150um) at
dipping material, dipping depth, and the process flow. The test reflow temperature, were open joints generated between the
matrix is seen in Table 5. bottom package and the test board. All of the solder joints
between the top and bottom packages were well formed. The
open joints were located at the corner of the package, which
Top Bottom Bottom Process Dipping Sample
was caused by the fact that the bottom package generated the
CSP CSP CSP flow depth size
concave warpage which parted the solder balls around the
package corners from the screen printed solder paste on the
warpage die size warpage
board at reflow temperature. In order to confirm this, the
warpage trend of the bottom package was monitored under
25um small 50um #2 150um 10
various temperatures as shown in Figure 6. In fact,
#3 150um 10
disconnection at corners was observed under high
middle 100um #2 150um 10 temperature.
#3 150um 10 The main reason open solder joints were observed only
large 150um #1 150um 10 between the bottom package and the test board and not
#2 150um 10
between packages may be explained as follows. Although the
#2 250um 10
bottom package showed large (150um) warpage with a large
#3 150um 10
die, the warpage of the area where the solder balls were
located was small. Using the shadow moiré measurements, it
was observed that while the area around the Cu pads on the
Table 5: Test matrix top side showed only a 50um warpage, the corresponding area
(solder balls) on the bottom side showed around a 100um
Practical Components, Inc.  Tel: 1-714-252-0010  Fax: 1-714-252-0026 11
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