A Study on Package Stacking Process for Package-on-Package (PoP) (cont.)
The package stacking was carried out by using the above- Screen print Process #1,2
Bottom CSP mount
Process #3 (solder dip)
Process #1 (flux dip)
Top CSP stack
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 mask on 0.28mm NSMD (Non Solder Mask Defined) pads of a test board. A conventional reflow profile with a 240C peak 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 example observed after the sample was peeled off.
Process #2, 3 (solder dip)
Reflow
Process
Screen print on board Dip bottom CSP to Place bottom CSP Dip top CSP to
Stack top CSP on bottom Reflow
#1 x
x
Flux x x
Table 4: Experimental process flow
Experimental procedure
As variables, we studied warpage of the bottom package,
dipping material, dipping depth, and the process flow. The test matrix is seen in Table 5.
Top
CSP warpage
CSP die size
Bottom Bottom Process Dipping Sample flow depth size
CSP warpage
25um small 50um #2 150um 10 #3 150um 10
middle 100um #2 150um 10 #3 150um 10
large 150um #1 150um 10 #2 150um 10 #2 250um 10 #3 150um 10
Table 5: Test matrix
#2 x
x
Paste x x
#3 x
Paste x
Paste x x
Figure 5: Open solder joints
Results and discussion
1. Influence by package warpage
The number of open solder joints when the warpage of the
bottom package is changed is shown in Table 6. It was observed that only in cases when the bottom package had a large die, meaning it displayed large warpage (150um) at reflow temperature, were open joints generated between the 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 was caused by the fact that the bottom package generated the concave warpage which parted the solder balls around the package corners from the screen printed solder paste on the board at reflow temperature. In order to confirm this, the warpage trend of the bottom package was monitored under various temperatures as shown in Figure 6. In fact, disconnection at corners was observed under high temperature. The main reason open solder joints were observed only
between the bottom package and the test board and not between packages may be explained as follows. Although the bottom package showed large (150um) warpage with a large 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 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
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