Flip chip assembly
The flip chip (dt. "Reversible assembly"), also known as controlled collapse chip connection (C4) is a method of mounting and connecting techniques ( AVT ) for contacting of bare semiconductor chips ( English bare the ) by means of contact bumps - so-called " bumps ".
With flip-chip assembly, the chip is mounted directly, without additional connecting wires, with the active contacting side facing down - towards the substrate / circuit carrier. Hence the name flip-chip ( to flip ). This leads to particularly small dimensions of the housing and short conductor lengths. In the case of very complex circuits, this technology often offers the only sensible connection option, because in some cases several thousand contacts have to be implemented. Thus, the entire area can of the be used for contacting, in contrast to wire bonding , where this is limited not possible or only very, because the wires cross and would most likely come into contact with each other. Furthermore, with wire bonding, the connections are made one after the other. With the flip-chip bonding technique, all contacts are connected simultaneously. That saves time.
In order to bond the chips , in addition to soldering and conductive gluing (see ICA and ACA ), pressure welding ( thermode bonding ) is also used as a joining process.
Other package designs are listed under chip housings .
C4 technology
C4 stands for the summary of the first letters of the terms " controlled collapsed chip connection " (= CCCC = C4).
C4 flip-chip technology was introduced by IBM in 1964 and has seen several modifications since then. This technique is used e.g. B. used in the manufacture of complex microprocessors . The production can be imagined as follows: The wafer is coated over its entire surface with a metal, e.g. B. by sputtering . A lacquer mask with defined openings is now applied . The solder is then electrodeposited . The lacquer mask is removed. This creates cylindrical solder bodies, as specified by the paint mask. These soldering cylinders form the contact points which establish the connection to the circuit structures in the deeper layers of the wafer or to each individual die. The remaining metal layer, which is not covered by the deposited solder, is removed using a selective etching process . The soldering cylinders are then melted into small balls ( bumps ) (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 creates an electrical connection between the contact surfaces of the chip and the contacts of the substrate ( housing , package ) ( reflow soldering ).
Another method of applying the bumps to a wafer is by stencil printing. The wafer, after it has obtained a solderable surface on the pads by means of electroplating, is printed with solder paste in a stencil printer. The wafer is then subjected to a reflow process, the solder paste melts and bumps arise. The wafer can then be cleaned to remove flux residues. This is followed by the separation of the chips and processing after they have been wetted with flux in the SMD manufacturing process.
After soldering onto the substrate (housing, “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 substrate do not destroy the structure.
After the underfill process, the flip chip will look like this:
Gluing with non-conductive adhesive (NCA)
The technology is often referred to as NCA (method non-conductive adhesive , dt. Non-conductive adhesive ), respectively. Here, the contacts of the chip are usually provided with so-called stud bumps. The stud bumps are made of gold wire. They are applied using the wire bond process ( ball-wedge process ) and then torn off directly above the ball. Some of the bumps are then flattened using a special tool and brought to an even height (coining). A non-conductive adhesive (usually based on epoxy) is applied to the substrate and the chip is pressed into it (bonded). The pressure during assembly must be high enough that the stud bumps safely pierce the adhesive layer in order to be able to establish an electrical connection. The adhesive is then cured at an elevated temperature, whereby the tool with the chip should be under constant pressure. For a secure connection, it is important that the adhesive shrinks when it dries, so that the bumps are drawn onto the contact surfaces of the substrate and an electrical contact is established.
Of all flip-chip assembly processes, NCA technology is best suited for small series , since the semiconductor chips can still be bumped after the wafer has been separated. The assembly process is also well suited for low quantities. The achievable contact distances are quite small, which means that a high level of integration is possible. The flat course of the adhesive under the chip eliminates the need for a separate underfill process.
Chip bonding under pressure leads to a relatively high cycle time , since the placement tool needs a certain cooling time so that the hardening process does not start too early for the next part. For large series, stud bumping is also not an ideal bumping process, as serial ball bonding requires considerably more time than flat application processes such as screen printing or sputtering.
Gluing with isotropic conductive adhesive (ICA)
This method is with ICA ( isotropic-conductive adhesive , dt. Isotropically conductive adhesive ), respectively. An isotropically conductive adhesive is applied to the contacts of the substrate . Then the chip with its contacts (with bumps) is placed on the adhesive dots. The adhesive is cured thermally or by means of UV radiation and thus creates a mechanical and electrical connection. Since the adhesive is not applied over the entire surface, an underfill is usually necessary after curing. In this process, the bumps are usually applied at the wafer level, e.g. B. by sputtering or vapor deposition of nickel. Stud bumps are possible, but are rarely used.
In contrast to the NCA or ACA method, the process does not have to run in series, which means that many chips can be cured in one pass. This leads to a reduced cycle time . The temperatures required for curing are generally lower than for soldering , so the thermal load on the component is lower.
This type of contacting is limited to a few and relatively large contacts, since the adhesive cannot be applied as finely as desired and also runs when the chip is placed. The process hardly offers any advantages over soldering flip chips, but it does require an additional process, while soldering is integrated into the assembly and connection technology (AVT) as a standard process. For these reasons, this procedure is rarely used .
Bonding with anisotropically conductive adhesive (ACA)
The method is referred to as ACA ( anisotropic conductive adhesive , dt. Anisotropically conductive adhesive ). Anisotropically conductive adhesive consists of an adhesive that is weakly filled with small conductive particles of the same size, e.g. B. gold coated polymer spheres. The adhesive is applied over the entire surface over the contacts of the substrate. Due to the low fill factor of the conductive particles, they are not connected after application, so that there is no conductive layer that would short-circuit the contacts. When the chip is placed, the adhesive is displaced by the mechanical pressure and the conductive particles are compressed until they are clamped between the bumps and the substrate pads and thus establish a conductive connection. To ensure a secure connection, the pressure is maintained during the curing process. As with the ICA process, the bumps are usually generated directly on the wafer.
One advantage is the surface gluing of the chip, so that an additional underfill process is no longer necessary. Furthermore, the achievable contact distances ( pitch ) are very small, significantly smaller than with the ICA process . The pressure required for loading is significantly lower than with the NCA process, which results in less mechanical stress.
As with the NCA process , one disadvantage is that the chip has to be pressed onto the substrate while the adhesive is hardening (thermocompression) so that the electrical contact is maintained, which has a negative effect on throughput. In addition, ACA glue is relatively expensive due to its complex structure and small number of manufacturers.
Web links
- Background to the Flip Chip Process ( Memento from March 15, 2016 in the Internet Archive ) - English
- Scientific study on flip-chip assembly on MID - (PDF; 8.3 MB)
- Research institute in the field of flip-chip technology