The Study on The Under Bump Metallurgy (Ubm) and 63SN-37PB Solder Bumps Interface for Flip Chip Interconnection
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250
Table I UBM structure and thickness
210 +/- 50C 183 C -- --- --0
20W
Structure
Thickness(pm)
"115
Al/Ti/Cu
1/0.2/5
T11
AI/TI/Cu
1/0.2/1
50
Nil Pdl
AVNi/Cu Al/Pd/Cu
1/0.2/1
nl
150
0 +/- l0se
100
'o
1/0.2/1 Fig.1
10
200 300400 TIMS(SeC)
Solder Reflow temperature profile
Al was used as an interconnection line between circuit elements on the Si chip. Because of the nonwettability of Pb/Sn on Al surface, Cu was used as a solderable layer. However, Cu has very low adhesion to any type of oxidized Al or SiO 2 surface, therefore, layers such as Ti, Ni and Pd were introduced as an adhesive layer between Cu and Al. From Pb/Sn solder wettability aspect, Ti is very poor, Ni is moderate and Pd is very good[2]. These barrier layers were also used as a diffusion barrier layer between Cu and Al. First two UBM structures -AlITi/5pm Cu (Ti5) and Al/Ti/lpim Cu (Til)- were designed to compare the Cu thickness effect. And the last three Al/Ti/l pm Cu (Til), A1/Ni/Ilpm Cu(Nil) and Al/Pd/lpm Cu(Pdl) - to compare the barrier layer effect. 280p.m pitch and 100pm size bumps were fabricated by electroplating method[4]. Fig.2 (a) shows patterned thick PR(photoresist), (b) shows mushroom shape solder bumps before solder reflow, (c) is 100pom size single solder bump after solder reflow process and (d) shows an SEM micrograph of area array solder bumps on a chip.
(a)
(b)
(c)
(d)
Fig.2 (a) Patterned Photoresist (b) Mushroom Shaped Solder Bump (c) Solder Ball after Reflow (d) Area array type solder bumps with 280p.m pitch and 100pm ball size Interfacial phenomena were investigated as a function of number of solder reflows and aging times at 150 0C. Solder reflows were conducted using a conventional solder reflow temperature profile shown in Fig. 1 in a three-zone infrared solder reflow oven. After the second solder reflow 68
which presents actual solder ball formation reflow step and subsequent flip chip bonding to a substrate step, samples were then aged isothermally in a convection oven at 150°C for 1, 2, 3, 10, 24 hours, and 7 days to simulate the underfill curing and also operating conditions. Scanning electron microscopy (SEM) and energy dispersive x-ray(EDX) were used to examine the crosssection microstructural morphology and composition of IMC layers at the interface between UBMs and solder. Phase of IMC layers were identified using a Rigaku X-ray diffractometer with Cu target. The mean thickness of the interfacial intermetallic layers was measured using the Global-Lab image process program. The fabricated eutectic Pb/Sn solder bumps have 100pm diameter size and 50pam diameter round UBM base. Ball shear strength of 30 solder bumps with various UBM structures and aging treatment were measured using the Dage 2400PC shear tester with a traveling speed of 380pam/sec. The stylus positioned approximately 5pam above the base Si surface to shear bumps off. 3. RESULTS A. Microstructual Observation Backscattered SEM images of UBM/solder interfaces which are four times solder reflow - (a), (b), (c), (d) - and two ti
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