Flip chip

Flip chip

The flip -chip assembly (Eng. " turn - assembly " ), also known as controlled collapse chip connection (C4) is a method of construction and connection technology (AVT ) for making contact of bare semiconductor chips (English bare die ) by Kontaktierhügel - so-called " bumps ".

In flip -chip mounting the chip directly, without connecting wires, with active bonding downwards - to the substrate / circuit board back - mounted. Hence the name flip- chip ( engl. to flip, turn ). This leads to very small dimensions of the housing and short conductor lengths. For very complex circuits this technology often provides the only meaningful connection ability, in part because several thousand contacts must be realized. Thus, the entire surface of the can be used for contacting, in contrast to wire bonding, where only a very limited, or is not possible, because the wires cross, and would most likely come into contact. Further, in wire bonding, the connections are made sequentially. In flip - chip bonding technique, the connection of all the contacts simultaneously. This saves time.

To bond the chip, in addition to soldering, and conductive adhesive bonding (see ICA and ACA) and pressure welding ( thermode bonding) is used as the joining method.

Other package types are listed under chip package.

C4 Technology

C4 represents the summary of the first letter of the terms "controlled collapsed chip connection" ( = CCCC = C4).

The C4 flip-chip technology was introduced in 1964 by IBM and underwent several modifications since then. This technique is used e.g. in the production of complex microprocessors. The production you can imagine as follows: The wafer is coated over the entire surface with a metal, eg, by sputtering. Now applying a resist mask with defined openings takes place. Then the solder is deposited galvanically. The resist mask is removed. This causes cylindrical solder body, as defined by the resist mask. This Lotzylinder form the contact points, which connect to the circuit patterns in the lower layers of the wafer or of each dies. With a selective etching the remaining, not covered by the deposited solder metal layer is removed. The Lotzylinder then into small balls ( bumps ) melted ( reflow ). Then, the wafers are separated into silicon chips. The chips are wetted with a flux, and the structure is heated so that the solder melts, and an electrical connection between the contact pads of the chip and the contacts of the substrate ( housing, package ) is prepared ( reflow soldering ).

Another method to provide a wafer with the bumps is the stencil printing. The wafer is after he has received a solderable surface on the pads by means of electroplating, printed in a stencil printer with solder paste. Thereafter, the wafer is subjected to a reflow process here, the solder paste melts and there are bumps in order. Then, the wafer may be cleaned to remove flux residues. It is followed by the separation of the chips and the processing after wetting with flux in the SMT manufacturing process.

After soldering the substrate (casing, " package " ), or the circuit board, the structure requires a so-called underfill ( an elastic, temperature-resistant plastic ), so that the different thermal expansion coefficients of the chip and the substrate does not destroy the structure.

After the underfill process of Flip Chip looks like this:

Bonding with non-conductive adhesive ( NCA)

The technology is often referred to as NCA method (non- conductive adhesive, dt not leifähiger adhesive). Here, the contacts of the chip are typically provided with so-called stud bumps. The stud bumps are made of gold wire. They are applied in the wire bonding method ( ball-wedge method ) and then pulled down directly over the ball. Some of the bumps are then flattened by using a special tool and brought to a uniform height ( Coining ). The substrate is a non-conductive adhesive (typically epoxy-based ) is applied, and the chip is pressed into ( bonded ). The pressure during loading must be sufficiently high that the stud-bumps to safely pierce produce the adhesive layer to provide electrical connection. Subsequently, the adhesive is cured under temperature, the tool should be compatible with the chip at a constant pressure. Is important for a secure connection, the adhesive shrinks upon drying, so that the bumps are grown on the contact areas of the substrate and an electrical contact is formed.

NCA technology is most suitable for all the flip -chip mounting method for small series, since the bumping of the semiconductor chips even after the separation of the wafer is possible. Also, the assembly process is good for low quantities suitable. The attainable contact distances are rather small, which means that a high integration is possible. By the two-dimensional development of the adhesive under the chip eliminates the need for a separate underfill process.

The chip bonding under pressure leads to a relatively high cycle time, since the Bestückwerkzeug a certain cooling time required for the curing process does not initiate too early the next part. For large series, the stud bumping is also not an ideal bumping process, since the serial ball bonding a lot more time than surface application methods such as screen printing or sputtering.

Bonding with isotropically conductive adhesive ( ICA)

This process is referred to as ICA ( isotropic - conductive adhesive, dt isotropic conductive adhesive ). On the contacts of the substrate, a conductive isotropic adhesive is applied. Then, the chip with its contacts is set ( with bumps) on the glue dots. The adhesive is cured thermally or by UV radiation, thus creating a mechanical and electrical connection. Since the adhesive is not applied all-over even a underfill is necessary after curing in the rule. The bumps are usually applied in this process at the wafer level, for example, by sputtering or vapor deposition of nickel. Stud bumps are possible, but are rarely used.

The process does not have to run serially, unlike the NCA or ACA method, ie there may be many chips are cured in one pass. This leads to a decreased cycle time. The temperatures required for curing are generally lower than in the soldering, the thermal load of the component is consequently lower.

This type of contact is limited to a few relatively large contacts, because the glue can not be applied arbitrarily fine and also runs during placement of the chip. The method offers little advantages over the soldering of flip- chips, but requires an additional process necessary during the soldering in the construction and connection technology ( AVT) is integrated as a standard process. For these reasons, this method is rarely used.

Bonding with anisotropic conductive adhesive ( ACA)

The process is called ACA (anisotropic - conductive adhesive, dt anisotropic conductive adhesive ). Anisotropic conductive adhesive consisting of an adhesive which is weakly filled with small conductive particles of the same size, such as gold-coated polymer spheres. The adhesive is applied over the entire surface of the substrate via the contacts. Due to the low filling factor of the conductive particles they are listed after the order not to contact, so there is no conductive layer, which would short circuit the contacts. When placing the chip, the adhesive is displaced by the mechanical pressure and the conductive particles compressed until they are trapped between the bumps and substrate pads and so establish a conductive connection. In order to ensure a secure connection, the pressure during the curing process is maintained. The bumps are usually created directly on the wafer as in the ICA process.

One advantage is a surface bonding of the chip, so that an additional underfill process is no longer necessary. Furthermore, the recoverable contact spacings ( pitch) are very small, much less than in the ICA method. The necessary pressure when fitting is clearly smaller than for the NCA procedures, which has a lower mechanical load to the row.

A disadvantage is, as in the NCA method, the chip has to be pressed to the substrate during curing of the adhesive ( thermal compression), so that electrical contact is maintained, which has a negative effect on the throughput. ACA is also relatively expensive due to its adhesive - complex structure and a small number of manufacturers.

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