DE102005041354B3 - Method of mounting components provided with gold stud bumps onto a circuit board using isostatic force - Google Patents

Abstract

The method involves immersing the gold stud bumps into a conductive adhesive, applying and aligning the components onto a contact region of the circuit board and hardening the adhesive. A settable filler is applied between the components and the circuit board and the filler is hardened. The circuit board with the filler and components is treated with a high isostatic pressure during the hardening process so that the component is pressed against the circuit board.

Description

The The present invention relates to a method of assembling a Gold PCB bumps printed circuit board by applying a curable Glue on one with gold stud bumps provided contact area of the printed circuit board, applying and aligning a circuit component on the adhesive, leaving the room between the printed circuit board with the gold stud bumps and the circuit component filled with glue is, and curing of the adhesive.

According to one Widely used flip-chip process technology (Stud Bump Bonding) isotropically more conductive Adhesive (ICA) for attaching a circuit component to the gold connectors (Gold stud bumps) of the printed circuit board used. The stability of the substrate or the printed circuit board (PCB) during drying the ICA and curing the underfill critical for good electrical connection of the flip-chip. This is Remember that the ICA because of the inequality of thermal expansion coefficients of substrate (PCB) and silicon chip is designed thermoplastic and therefore has only a limited adhesive power. PCBs, to a greater extent internal forces save, give these powers while increased Drying or curing temperatures free. this leads to to curvatures or relative movements between the silicon chip and the PCB, which is microscopic jumps in which ICA can cause. Thus, open circuit connections result, and consequently an increased scrap in the production.

at The current flip-chip manufacturing process becomes a cover plate and a base plate that magnetically attract each other. In between, the printed circuit board or PCB is introduced, which thereby is kept flat in all process steps. This method However, there are limitations to the fact that PCBs usually not completely flat structures possess and the cover plate on the places openings must have, at which silicon chips are attached. Therefore, a slight movement of the PCBs during temperature increases is always possible. In critical cases In the process, the scrap is increased by 10 to 15% during production.

To the Harden of the adhesive held or fixed the silicon chips on the circuit boards become. However, a mechanical hold down of the silicon chips is not possible, because it increased Risks of misalignments of silicon chips are given.

In In this context, Matsushita has proposed an air pressure solution, after which a special machine made a thin, readily yielding Movie over the flip-chip arrangement lays. When air pressure directional from above is applied, presses the thin one Film the chips against the PCB so that it is pressed down firmly and possibly also a curvature of the PCBs during of curing is contained. However, this approach has disadvantages when mechanical cover plates with very small openings for silicon chips be used, which may result in a so-called "tent effect", the Benefit of this approach reduced. In addition, at the there presented solution continues a mechanical holding step for the application of the flip chip necessary to a springback to prevent the silicon chip when it is mounted with some pressure becomes.

you could Therefore, to conclude, instead of the gold-stud-bump flip-chip technology the so-called solder bump flip-chip technology, the a reflow soldering (reflow soldering) involves using. The reflow soldering process is very expensive.

From the publication US 2002 09 4671 A For example, a method of populating a printed circuit board by reflow soldering is known. Solder bumps lead to a gap between the flip chip and the substrate of the printed circuit board. To seal this space, a potting compound is applied to all sides of the flip chip. With isostatic pressure, the potting compound is pressed into the gap and around the solder bumps. Subsequently, the potting compound is cured.

From the publication EP 0 724 289 A2 For example, a method of manufacturing a semiconductor package according to the preamble of claim 1 is known. The introduction of the underfill is carried out under lower pressure than the hardening of the underfill. This is done for the purpose of faster introduction.

Furthermore US 2002/0048847 A1 discloses a method for Attaching a semiconductor chip on a substrate. The curing of the Adhesive and the underfill take place here in the same step.

Further Method in which a pressing of the circuit component by Pressure differences takes place, are from the documents US 2003/0027371 A1 and US 2001/0015010 A1.

Furthermore, the document describes EP 0 824 270 B1 a method for mounting multiple Elek Audio components on a circuit board. This placement process by means of isostatic pressure is carried out in particular in an autoclave.

The The object of the present invention is therefore the method for loading a printed circuit board with gold stud bumps to improve the efficiency.

According to the invention this Task solved by a method for loading a printed circuit board with a gold Stud Bumps circuit component, in particular flip chip, through Dip the gold stud bumps into a conductive adhesive, applying and aligning the circuit device on a corresponding contact area the printed circuit board, curing of the conductive Adhesive, introducing a curable underfilling between the circuit component and the printed circuit board, curing the Underfilling, and loading the printed circuit board including the underfill and of the circuit component with respect to the normal ambient pressure elevated, isostatic Pressure during of curing, so that the circuit device with the gold stud bumps is pressed against the printed circuit board.

Of the Invention is based on the idea, the above-mentioned technical Problem with the help of an isostatic pressure coming from all directions or Pages is equal to use, a movement of the flip chip to minimize on the substrate. While the substrate, i. H. the Printed circuit board, and the flip-chip be subjected to the isostatic pressure, so that the flip-chip With the gold stud bumps pressed onto the substrate, the underfill is cured.

Of the The advantage of using isostatic pressure is that that the applied pressure is uniform in all directions, so that no shifting of the silicon chip, eventually leading to misalignment to lead would, can result. Due to the larger surface of the Chip top compared with its side surfaces where the flanks of the underfilling There is a resultant force on the silicon chip down while the isostatic pressure is applied. This pushes the silicon chip against the substrate when the underfill cures and pushes Furthermore the substrate against the retaining base plate. This is another Effect, namely that no curvature of the substrate can be done, which is not microscopic jumps of the ICA, which are due to possible curvatures of the substrate, result.

If about that the silicon chip is pushed as close as possible to the substrate, The ICA is between the gold stud bump tip and the substrate further pressed together, so that in some cases the Gold Stud bump directly to the pad of the substrate in touch occurs. this leads to to a lower contact resistance per bump.

It can also be observed that the underfill edges closer to the Pressed edges of the silicon chip become. This is especially for products with SMD pads that are very close to the silicon chip, helpful. These SMD pads are allowed not from the underfilling, that may be out tipped out of the silicon chip, covered because this would lead to problems of the subsequent SMD process.

Preferably is the isostatic pressure between 1 and 10 bar. Very good results in terms of manufacturing quality can be with an isostatic Reach pressure of 2 bar.

Of the isostatic pressure presses the circuit component onto the printed circuit board. For this purpose, the isostatic pressure at the beginning of curing or before the curing process gradually or gradually increased with very small steps. This can be avoided be that due to a pressure jump shifting the flip chip takes place on the substrate or the printed circuit board. Especially advantageous is when the underfill is thermosetting and the isostatic pressure with simultaneous heating of the underfilling for curing is exercised. This can easily create a permanent and stable connection be made between the printed circuit board and the flip-chip.

conveniently, becomes the method according to the invention for loading a printed circuit board in an autoclave. In it can be build up in a known manner the necessary isostatic pressure and if necessary, also set the necessary temperature.

The The present invention will now be explained in more detail with reference to the accompanying drawing which the production process according to the invention sketchy.

The below described embodiment represents a preferred embodiment of present invention.

In the FIG a printed circuit or a printed circuit board L is shown, the between an aluminum base plate A and steel cover plate S a is added. The steel cover plate S has in a plan view of a rectangular recess AS, which keeps free to be mounted on the printed circuit board L flip-chip FC. For the sake of intuition, there is a gap between the aluminum base plate A and the printed circuit board L and between the steel cover plate S and the printed circuit board L in the FIG. In practice, however, these components touch each other.

In the aluminum base plate A magnets are incorporated in the FIG are not shown. These magnets pull the steel cover plate down and hold the PCB L thus for the assembly process flat on the aluminum base plate A.

A underfilling U is in the small gap between the flip-chip FC and the printed circuit board L brought. This gap is approximately 50 μm high and corresponds to the height of the gold stud bumps GB of the flip chip FC. The gold stud bumps GB are with unillustrated connection points the printed circuit board L using an isotropic conductive adhesive connected. Since in practice the electrical connection points of the PCB L in height to each other by up to about 20 microns There is always a risk that a gold stud bump will not vary is contacted when the conductive Adhesive only one height of about 10 μm reached.

The underfilling U is in its uncured state a low viscosity liquid. After curing at about a temperature of 150 ° C it reduces its volume. This shrinking pulls the flip chip FC to the circuit board L, causing the gold stud bumps GB on the connection points the printed circuit board L or the conductive adhesive therebetween depressed become. This is the main mechanism for everyone single gold stud bump GB a connection with low contact resistance manufacture.

For curing is now held between the plates A and S printed circuit board L in introduced an autoclave, not shown in the FIG. such Autoclaves are used, for example, for curing composite materials such as carbon fibers, kevlar, etc. used. Isostatic pressure is symbolized in the FIG with arrows ID. He works evenly from all sides on the individual components in the autoclave. This Print ID from all sides leads not only to make the printed circuit board L tighter between the two Plates A, S is held, but also causes the flip chip FC to the circuit board L pressed becomes. Of course A lateral pressure also acts on the four sides of the flip chip FC. This lateral pressure is pressing the underfill U continues under the flip-chip FC, as long as it is still liquid. This results in an upward force, which tries to push the flip-chip FC upwards. On the bottom of the flip chip FC, isostatic pressure can not immediately exert a force. There now the entire vertical surface along the flip chip FC much smaller than the horizontal area at the top of the flip-chip FC, there is a net resultant force which pushes the flip-chip FC down.

In the autoclave thus becomes the flip-chip FC all the time pressed down, until the underfill U is cured by the heater in the autoclave. After that it stays State of compression by the hardened underfill U is constantly maintained.

Of the The advantage of isostatic pressure is not just that of the flip chip FC is pressed with its gold stud bumps GB against the printed circuit board L, but that the flip chip FC is also in the xy direction at its Place is held. This is important because until complete curing of the underfill U any slight movement of the flip chip FC in the xy direction to one Misalignment of the flip-chip FC on the printed circuit board L can lead. For this reason, it is also necessary to increase the pressure in the autoclave not easy to turn on, but slowly increase it. Out for the same reason, it is not advisable to apply the pressure mechanically, because this is just the risk that the flip-chip offset becomes.

One Another advantage of the autoclave, the above briefly mentioned is that the printed circuit board L is absolute during cure of the underfill kept flat. This is all the more necessary because of practical establish only as strong magnets are used in the aluminum base plate, that the cover plate can be removed by hand. It also takes the magnetic strength at high temperatures. Will the printed circuit boards L then only held with the magnets, would some with stronger ones enclosed internal pressures due to their multi-layer structure try the respective pressure at higher Release temperatures, which leads to so-called micro-dislocations. These Microcracks crowd under circumstances the gold stud bumps from the connector points of the printed circuit board L away, causing the conductive Adhesive in the cured Condition is broken, leaving an electrical interruption results. With the additional Applying the isostatic pressure on the steel cover plate S can cause these micro-distortions be prevented, whereby the committee caused thereby not is to be feared.

The isostatic pressurization results in the further advantage that when the underfill U below the flip-chip FC is compressed prior to curing, all air bubbles, the remain in the later cured state corresponding clearances are pushed from the liquid or compressed.

In Advantageously, in the assembly also on the so-called "vacuum Bagging "waived be, with the mechanical pressure on laminates and the like exercised shall be.

Claims (6)

Method for assembling a printed circuit board (L) with a gold stud bumps (GB) circuit component (FC) through - Diving gold stud bumps (GB) into a conductive adhesive, - Apply and aligning the circuit device (FC) with a corresponding one Contact area of the printed circuit board (L), Curing the conductive adhesive, - bring in a curable underfilling (U) between the circuit component (FC) and the printed circuit board (L), - curing the underfilling (U), marked by - Applying pressure to the printed circuit board (L) including the underfilling (U) and the circuit component (FC) with respect to the increased normal ambient pressure, isostatic pressure (ID) during of curing, so that the circuit component (FC) with the gold stud bumps (GB) pressed against the printed circuit board (L) becomes. The method of claim 1, wherein the isostatic Pressure (ID) is between 1 and 10 bar. The method of claim 1, wherein the isostatic Pressure (ID) is 2 bar. Method according to one of the preceding claims, wherein the isostatic pressure (ID) gradually up to a predetermined final value at the beginning of curing or before the curing process elevated becomes. Method according to one of the preceding claims, wherein the isostatic pressure (ID) with simultaneous warming of the underfilling (U) for curing is applied. Method according to one of the preceding claims, wherein applying isostatic pressure (ID) and curing in an autoclave takes place.