Adapted assembly processes for flip-chip technology with solder bumps of 50 &amp;#x00B5;m or 40 &amp;#x00B5;m diameter

Härter

Goßler

et al. 2011

Self Cite

This tutorial examines the reliability of ultra fine-pitch flip chip assemblies with lead-free solder bumps for which there are two major reliability aspects to consider: Thermo cycling performance and electromigration (EM). Isothermal aging and humidity should not cause significant problems, providing that suitable materials have been selected, see [1] and chapter "Results". First, dry heat storage, temperature humidity bias testing, and temperature cycling were conducted, showing excellent reliability of these ultra fine-pitch assemblies. Second, electromigration (EM) performance of the flip chip assemblies (with solder sphere sizes mentioned above) was investigated. Experiment The test coupons used in this study are specially designed flip chip packages. SAC305 Solder spheres with 40 µm, 50 µm, or 60 µm diameter were attached to silicon chips 10 x 10 x 0.8mm in size, using the wafer level solder sphere transfer process (often called "gang ball placement") and 30 µm SAC305 spheres using a laser based single-sphere process, respectively [2]. Even exotic solder compositions are readily accommodated using these methods.

Section: Discussionmentioning

confidence: 99%

Section: Figmentioning

confidence: 99%

Accelerated Life Tests of Flip-Chips With Solder Bumps Down to 30 μm Diameter

Härter

Goßler

et al. 2011

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“…The test samples and their fabrication have been described in detail in [2][3][4]. Specially designed, 10 mm x 10 mm x 0.8 mm large silicon chips were assembled onto thin film ceramic (Al 2 O 3 ) substrates by automatic placement and consecutive reflow soldering.…”

Section: Methodsmentioning

confidence: 99%

Study on electromigration in flip chip lead-free solder connections with 40 μm or 30 μm diameter on thin film ceramic substrates

Gorywoda

Kandler

et al. 2015

Electromigration comprises one of the processes affecting the long-term reliability of electronic devices; it has therefore been the focus of many investigations in recent years. In regards to flip chip packaging technology, the majority of published data is concerned with electromigration in solder connections to metallized organic substrates. Hardly any information is available in the literature on electromigration in lead-free solder connections on thin film ceramic substrates. This work presents results of a study of electromigration in lead-free (SAC305) flip chip solder bumps with a nominal diameter of 40 μm or 30 μm with a pitch of 100 μm on silicon chips assembled onto thin film Al2O3 ceramic substrates. The under bump metallization (UBM) comprised of a 5 μm thick electroless nickel immersion gold (ENIG) layer directly deposited on the AlCu0.5 trace. The ceramic substrates were metallized using a thin film multilayer (NiCr-Au(1.5 μm)-Ni(2 μm) structure on the top of which wettable areas were produced with high precision by depositing flash Au (60 nm) of the required diameter (40 μm or 30 μm). All electromigration tests were performed at the temperature of 125 °C. Initially, one chip assembly with 40 μm and one with 30 μm solder bumps was loaded with the current density of 8 kA/cm2 for 1,000 h. The assemblies did not fail and an investigation with SEM revealed no significant changes to the microstructure of the bumps. Thereafter seven chip assemblies with 40 μm solder bumps and five assemblies with 30 μm bumps were subjected to electromigration tests of 14 kA/cm2 or 25 kA/cm2, respectively. Six of the 40 μm-assemblies failed after 7,000 h and none of the 30 μm-assemblies failed after 2,500 h of test duration so far. Investigation of failed samples performed with SEM and EDX showed asymmetric changes of microstructure in respect to current flow. Several intermetallic phases were found to form in the solder. The predominant damage of the interconnects was found to occur at the cathode contact to chip; the Ni-P layers there showed typical columnar Kirkendall voids caused by migration of Ni from the layers into the solder. Failure of the contacts apparently occurred at the interface between Ni-P and solder. In summary, the results of the study indicate a very high stability of lead-free solder connections on ceramic substrates against electromigration. This high stability is primarily due to a better heat dissipation and thus to a relatively low temperature increase of the ceramic packages caused by resistive heating during flow of electric current. In addition, the type of the metallization used in the study seems to be more resistant to electromigration than the standard PCB metallization as it does not contain a copper layer.

“…An automatic Flip Chip Bonder with a placement accuracy of ± 10 µm @ 3s was used for the assembly. For the assembly experiments different substrates were employed, FR4/BT [1] and thin film ceramic [26]. Figure 5a and b show the test vehicle, a 10mm x 10mm Flip Chip with an electroless UBM, and 100µm pitch on a thin film ceramic.…”

Section: Flip Chip Assemblymentioning

confidence: 99%

Wafer Level Solder Bumping and Flip Chip Assembly with Solder Balls Down to 30μm

Oppert¹,

Franke³

et al. 2011

Self Cite

The most important technology driver in the electronics industry is miniaturization mainly driven by size reduction on wafer level and cost. One of the interconnection technologies for fine pitch applications with the potential for highest integration and cost savings is Flip Chip technology. The commonly used method of generating fine pitch solder bumps is by electroplating the solder. This process is difficult to control or even impossible if it comes to ternary or quaternary alloys. The work described in this study addresses the limitations of existing bumping technologies by enabling low-cost, fine pitch bumping and the use of a very large variety of solder alloys. This flexibility in the selection of the solder materials and UBM stacks is a large advantage if it is essential to improve temperature cycling resistance, drop test resistance, or to increase electromigration lifetime. The technology allows rapid changeover between different low melting solder alloys. Tighter bump pitches and a better bump quality (no flux entrapment) are achievable than with screen printing of solder paste. Because no solder material is wasted, the material costs for precious metal alloys like Au80Sn20 are much lower than with other bumping processes. Solder bumps with a diameter between to date 30 μm and 500 μm as well as small and large batches can be manufactured with one cost efficient process. To explore this potential, cost-efficient solder bumping and automated assembly technologies for the processing of Flip Chips have been developed and qualified. Flip Chips used in this study are 10 mm by 10 mm in size, have a pitch of 100 μm and a solder ball diameter of 30 μm, 40 μm or 50μm, respectively. Wafer level solder application has been done using wafer level solder sphere transfer process or solder sphere jetting technology, respectively. The latter tool has been used for many years in the wafer level packaging industry for both Flip Chip and chip scale packaging applications. It is commonly known in the industry as a solder ball bumping equipment. For the described work the process was scaled down for processing solder spheres with a diameter of 30 μm what was never done before that way worldwide. The research has shown that the underfill process is one of the most crucial factors when it comes to Flip Chip miniaturization for high reliability applications. Therefore, high performance underfill material was qualified initially [1]. Final long term reliability testing has been done according to MIL-STD883G, method 1010.8, condition B up to thirteen thousand cycles with excellent performance of the highly miniaturized solder joints. SEM/EDX and other analysis techniques will be presented. Additionally, an analysis of the failure mechanism will be given and recommendations for key applications and further miniaturization will be outlined.