IMPACT 2017

Press-fit connection is a solderless technology for making reliable electronic joints. Modern press-fit compliant solution provides many advantages when compared with soldering, such as solderless, low thermal stress, better reliability, lower cost, etc., making it a popular interconnect technology. It is reported that press-fit connectors have begun to dominate high-end connector applications for telecommunications, computer printed circuit boards (PCBs) and automotive electronic modules. The press-fit technology allows for the insertion of a press-fit pin (male) into a plated-through hole (PTH, female) in a printed circuit board (PCB) to establish a cold welded interconnections autonomously without using solder, especially when tin plating is used for at least one of both contact partners (pin/hole). The press-fit compliant pins having an elastic press-in zone deform when inserted into a PTH and sustain a permanent high contact normal force between the compliant pin and PTH. The external pressure applied by the compliant press-fit zones during and after performing the press-in process increases the tendency to create tin whiskers especially for pure tin finish. These whiskers grow on much shorter time scales than whiskers caused by the stress introduced by intermetallic phase growth. Furthermore, the technology trend towards higher density connector solutions (closer pin to pin/hole to hole distance, much higher external pressure) further increases the risk of electrical short-circuit due to the formation of tin whiskers.

In this study, a series of factors that may affect whisker growth on electroplated tin coatings under external pressure were studied, including storage time, coating thickness, grain structure, loading, ball diameter, etc., to improve the understanding of the external pressure-induced whisker growth mechanism. It was found that longer storage time, thicker coating (< 3 μm), higher loading and higher pressure will give more and longer whiskers. Equiaxed grain structure with moderate grain size can disperse external compressive stress more efficiently, thus mitigating whisker growth

The new product provides via filling with higher throwing power in through holes while maintaining high plating rates, enabling high production throughputs. The plating process was evaluated as a function of a variety of process variables including eductor distance, solution flow rate, current density and additive concentrations. The test panels used were 1mm thick with 60 µm deep vias on one side and 100 µm deep vias on the other side. The panels included through holes of 0.15mm and 0.25 mm diameters and micro-via diameters between 75-150 µm. The test panels were processed through Electroless Copper prior to electroplating which was performed in 60 Liter vertical in-line plating equipment using insoluble anodes.

An optimum flow rate of 25 L/min was used for better via fill and throwing power. The eductor was positioned at 1, 2 and 3 inches away from the test panel. The optimum distance was determined to be 1 inch to achieve filled vias with dimples less than 10 µm. Plating tests were also carried out as a function of current density (15, 20, 25 and 30 ASF). Dimple depth gradually increased with increasing current density from 15 to 30 ASF but still remained below 10 µm in depth. Throwing Power greater than 80% was achieved in 0.15 mm and 0.25 mm diameter through holes. The plating bath was cycled to 200 Ah/L to evaluate the performance.

This paper will discuss the development of a novel next generation product for micro-via fill (60 µm and 100 µm deep vias). Dimple depths less than 10 µm, void-free fill at a surface Copper thickness of less than 21 µm can be achieved with this next generation product.

Previously we have introduced the innovative nano-silver catalyst system such as the physical properties, catalytic performance on conventional electroless copper deposition, and reliability performance in copper-copper interconnects area. [1] In this paper, comparison of silver catalyst with traditional palladium catalyst for MHC process will be described. Further study on deposition profile, microvia coverage and morphology, and reliability performance with liquid-to-liquid thermal shock will be illustrated. In the last section, the concerns on silver migration with silver catalyst applications on printed circuit board will be discussed.

The Immersion Gold (IG) bath provides the protective Au layer after the Electroless Ni (EN) process following the below equation: 2Au+ + Ni → 2Au + Ni2+. As an electrochemical process, the deposition rate (or Au thickness per unit of time) is highly dependent on the redox potential of Au+ and electropotential of the Ni surface. The former can be considered as a fixed value in a Au plating solution while the latter will vary depending on the design of boards (pad size, connection of pad to the inner-layer Cu, etc.). The differences of surface potential, or so-called galvanic effect, makes the deposition of Au non-uniform, which may influence the properties of the board in many aspects. For example, chromatic aberration of the Au surface appears and some of the extremely thick Au pads may appear as red color; solderability may become uneven across all pads; and Au may be wasted due to the poor gold thickness distribution. More seriously, the galvanic effect may cause Au skip-plating, or black Ni because of Ni over-dissolution on specific pads, resulting in the solderability problem.

Our new immersion gold technology can effectively reduce the galvanic effect by introducing a new chemical system. The electropotential differences were minimized among pads with various surface areas or connected to different inner-layer copper areas thus providing a uniform gold deposition. Meanwhile, the plated Au thickness can be steadily increased from less than 1 microinch to 3 microinches and metallic lustre of the Au surface remained normal, from the pale yellow of thinner Au to the lemon yellow of thicker Au.

As green material and process safety are the future trend in the industry, development of cyanide free immersion gold process is one of remarkable research topics. However, up to now, there is only limited amount of cyanide free immersion gold bath available in the market. The challenge is to maintain good bath stability with continuous operation when getting rid of cyanide to complex the gold ions.

In this paper, a novel cyanide free immersion gold bath will be described which can be applied in ENIG process for continuous operation. Beside free of cyanide, the bath characteristic can be operated at about 45°C and neutral pH. This operating temperature is much lower than traditional cyanide containing immersion gold bath which operated at about 80°C. This bath can solve the EHS concerns and serve for energy saving as well. The bath was stable with no significant change in plating rate after long time idling. Most importantly, the plating performance is equivalent to conventional immersion gold bath.