Warpage Control With Intermediate Material

Abstract

A mounting device for mounting electronic components, wherein the mounting device comprises an electrically conductive structure having a first value of thermal expansion in at least one pre-defined spatial direction, an electrically insulating structure having a second value of thermal expansion in the at least one pre-defined spatial direction being different from the first value and being arranged on the electrically conductive structure, and a thermal expansion adjustment structure having a third value of thermal expansion in the at least one pre-defined spatial direction, wherein the third value is selected and the thermal expansion adjustment structure is located so that thermally induced warpage of the mounting device resulting from a difference between the first value and the second value is at least partially compensated by the thermal expansion adjustment structure.

Claims

1. A mounting device for mounting electronic components, wherein the mounting device comprises: an electrically conductive structure having a first value of thermal expansion in at least one pre-defined spatial direction; an electrically insulating structure having a second value of thermal expansion in the at least one pre-defined spatial direction being different from the first value and being arranged on the electrically conductive structure; a thermal expansion adjustment structure having a third value of thermal expansion in the at least one pre-defined spatial direction; wherein the third value is selected and the thermal expansion adjustment structure is located so that thermally induced warpage of the mounting device resulting from a difference between the first value and the second value is at least partially compensated by the thermal expansion adjustment structure; wherein the third value is selected to be smaller than both the first value and the second value.

2. The mounting device according to claim 1, configured as a plate with a thickness extending in z-direction and having a length in x-direction and a width in y-direction being both larger than the thickness.

3. The mounting device according to claim 2, wherein the at least one predefined spatial direction comprises at least one of the x-direction, the y-direction, and the z-direction.

4.-6. (canceled)

7. The mounting device according to claim 1, wherein the thermal expansion adjustment structure comprises or consists of one of the group consisting of diamond like carbon, a nitride, and an oxide.

8. The mounting device according to claim 1, wherein the electrically conductive structure the electrically insulating structure, and the thermal expansion adjustment structure constitute a layer stack.

9. The mounting device according to claim 1, wherein the thermal expansion adjustment structure is embedded between different spatially separated sections of the electrically conductive structure.

10. The mounting device according to claim 1, further comprising an adhesion promoting structure arranged between the thermal expansion adjustment structure and the electrically conductive structure and configured for promoting adhesion of the thermal expansion adjustment structure on the electrically conductive structure.

11. The mounting device according to claim 10, wherein the mounting device comprises at least one of the following features: the adhesion promoting structure extends over only a part of the surface of or over the entire surface of at least one of the electrically conductive structure and the thermal expansion adjustment structure; the adhesion promoting structure comprises or consists of at least one of the group consisting of titanium, tungsten, chromium, a carbide builder, a carbon composite, graphene, and a monolayer graphene.

12. (canceled)

13. The mounting device according to claim 1, wherein the electrically conductive structure comprises two spatially separated sections arranged on two opposing sides of the thermal expansion adjustment structure, and the electrically insulating structure comprises two spatially separated sections arranged on two opposing sides of the two sections of the electrically conductive structure.

14. The mounting device according to claim 13, wherein the two sections of the electrically conductive structure, the two sections of the electrically insulating structure, and the thermal expansion adjustment structure form a symmetric arrangement.

15. The mounting device according to claim 14, wherein the mounting device comprises at least one of the following features: adjustment structure is made of a thermally conductive and electrically insulating material; the thermal expansion adjustment structure is made of a material having a value of the thermal conductivity of at least 2 W/m K.

16. (canceled)

17. The mounting device according to claim 1, wherein the electrically insulating structure is made of a material having anisotropic properties in terms of thermal expansion.

18. The mounting device according to claim 1, comprising at least one of the following features: at least one further electrically conductive structure; at least one further electrically insulating structure on the electrically insulating structure and the electrically conductive structure.

19. (canceled)

20. The mounting device according to claim 1, configured as one of the group consisting of a printed circuit board, an interposer, a substrate, and a multilayer substrate.

21. The mounting device according to claim 20, wherein the mounting device comprises at least one of the following features: the thermal expansion adjustment structure is configured so that the thermally induced warpage is characterized by a bow of the mounting device of at the maximum 1.5%, wherein the bow is calculated as a ratio between a maximum distance between a bottom surface of the mounting surface and a planar support carrying the mounting device on the one hand and a length of a longest side of the mounting device on the other hand; the thermal expansion adjustment structure is configured so that the thermally induced warpage is characterized by a twist of the mounting device of at the maximum 2%, wherein the twist is calculated as a ratio between a maximum distance between a corner of the mounting surface and a planar support carrying the mounting device on the one hand and a longest diameter of the mounting device on the other hand; the thermal expansion adjustment structure is configured as a partial layer having, in a viewing direction perpendicular to a main surface of the mounting device, a total area being smaller than a surface area of the main surface of the mounting device.

22.-23. (canceled)

24. A method of manufacturing a mounting device for mounting electronic components, wherein the method comprises: providing an electrically conductive structure having a first value of thermal expansion in at least one pre-defined spatial direction; arranging an electrically insulating structure on the electrically conductive structure, the electrically insulating structure having a second value of thermal expansion in the at least one pre-defined spatial direction being different from the first value; forming a thermal expansion adjustment structure having a third value of thermal expansion in the at least one pre-defined spatial direction; selecting the third value and locating the thermal expansion adjustment structure so that thermally induced warpage of the mounting device resulting from a difference between the first value and the second value is at least partially compensated by the thermal expansion adjustment structure; wherein the third value is selected to be smaller than both the first value and the second value.

25. The method according to claim 24, wherein the thermal expansion adjustment structure is formed by one of the group consisting of physical vapor deposition, cathodic arc deposition, chemical vapour deposition, plasma enhanced chemical vapour deposition, and printing.

26. The method according to claim 24, wherein the third value is selected so that the electrically conductive structure and the thermal expansion adjustment structure as a composite have an effective value of the thermal expansion in the at least one pre-defined spatial direction which is closer to the second value than the first value.

27. The method according to wherein the method comprises: estimating thermally induced warpage of a mounting device without a thermal expansion adjustment structure; estimating thermally induced warpage of a to-be-designed-mounting device having a thermal expansion adjustment structure; modifying the third value of the thermal expansion adjustment structure and/or modifying location of the thermal expansion adjustment structure in the to-be-designed-mounting device until an estimated thermally induced warpage of the to-be-designed-mounting device meets at least one predefined quality criterion in term of warpage characteristics; manufacturing the to-be-designed-mounting device meeting the at least one predefined quality criterion.

28. The method according to claim 24, wherein the thermal expansion adjustment structure is formed of diamond like carbon, wherein a mixture of sp.sup.2 and sp.sup.3 hybridized carbon of the thermal expansion adjustment structure is adjusted so that the thermally induced warpage of the mounting device resulting from the difference between the first value and the second value is at least partially compensated by the thermal expansion adjustment structure.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Embodiments of the invention will be described in more detail hereinafter with reference to examples of embodiment but to which the invention is not limited.

[0046] FIG. 1 illustrates a cross sectional view of a mounting device according to an exemplary embodiment of the invention.

[0047] FIG. 2 illustrates a cross sectional view of a mounting device according to another exemplary embodiment of the invention.

[0048] FIG. 3 illustrates a cross sectional view of a mounting device according to yet another exemplary embodiment of the invention.

[0049] FIG. 4 illustrates a phase diagram indicating contributions of sp.sup.2 hybridized carbon, sp.sup.3 hybridized carbon and hydrogen of a carbon comprising thermal expansion adjustment structure of a mounting device according to an exemplary embodiment of the invention, wherein mechanical and thermal properties of the mounting device may be adjusted by configuring a manufacturing procedure in accordance with a desired section of the phase diagram.

[0050] FIG. 5 illustrates a three-dimensional view of a mounting device according to an exemplary embodiment of the invention having a bow-type warpage of a still acceptable extent and lying on a flat support.

[0051] FIG. 6 illustrates a three-dimensional view of a mounting device according to an exemplary embodiment of the invention having a twist-type warpage of a still acceptable extent and lying on a flat support.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0052] The illustrations in the drawings are schematical. In different drawings, similar or identical elements are provided with the same reference signs.

[0053] Before exemplary embodiments will be described in further detail referring to the figures, some general considerations of the present inventors will be presented based on which exemplary embodiments have been developed.

[0054] As miniaturization of substrates and especially thickness reduction are further going on, uncontrolled bow and twist due to different coefficients of thermal expansion (CTE) values of used materials in a build-up may occur. This warpage leads in following processes, like assembly, to registration problems, misplacing of components, adhesion problems while stencil printing, etc. Therefore, a balanced build up and an equal amount of used materials are advantageous to avoid such problems caused by various CTE values. According to an exemplary embodiment of the invention, one or more thermal expansion adjustment structures (such as intermediate layers) with low CTE values act in such build-ups as a stiffener layer and distribute the expansion force over the whole substrate area while other materials with a higher CTE value are forced to stay in form. Finding a compromise between all used materials to maintain functionality and control warpage is one possible target in this context.

[0055] According to an exemplary embodiment, at least one intermediate layer as thermal expansion adjustment structure is deposited via various processes (such as PVC, ARC, printing technologies, CVD) on certain inner copper layers or other electrically conductive structures of multilayer substrates. The layer forming the thermal expansion adjustment structure can be on the full area or just on specific areas, with masking the areas, of the substrate. Warpage control may be realized due to the selection of a low CTE value of the intermediate layer in a defined thickness. The for instance layer-shaped thermal expansion adjustment structure can be any material like amorphous carbon material, nitrides or oxides or a stoichiometric or non-stoichiometric mixture of these and/or other materials.

[0056] When CTE values of exemplary materials are compared: FR4 (as an example for a material for the electrically insulating structure) shows the following values of thermal expansion (wherein the x- and the y-axis are mainly influenced by glass fibres): [0057] CTE—x-axis: 14 10.sup.−6 K.sup.−1 [0058] CTE—y-axis: 12 10.sup.−6 K.sup.−1 [0059] CTE—z-axis: 70 10.sup.−6 K.sup.−1

[0060] Copper (as an example for a material of the electrically conductive structure): [0061] CTE: 16 10.sup.−6 K.sup.−1

[0062] DLC (as an example for a material for the thermal expansion adjustment structure): [0063] CTE: <4 10.sup.−6 K.sup.−1

[0064] Hence, the CTE values of copper and FR4 are significantly different and this causes bow and twist. According to an exemplary embodiment of the invention, such problems are suppressed or even fully eliminated by placing a thermal expansion adjusting material in between such layers of copper and FR4 which balances the different expansion, and consequently the effect of bow and twist. Copper does not adhere properly on DLC so an adhesion promoter like Ti, W other carbide builder or carbon composites may be used and may be deposited also with PVD, ARC, CVD or similar processes.

[0065] Hence, exemplary embodiments of the invention provide a warpage control to realize asymmetric build-ups and as a consequence of very thin substrates. Embodiments of the invention may be implemented particularly advantageously for providing substrates with embedded components and for very thin substrates.

[0066] FIG. 1 illustrates a cross sectional view of a mounting device 100 according to an exemplary embodiment of the invention.

[0067] The mounting device 100 is here embodied as a printed circuit board (PCB) and is configured for mounting electronic components (not shown) on one or both opposing main surfaces thereof. As can be seen in FIG. 1, the mounting device 100 is configured as a multilayer substrate or plate with a thickness extending in z-direction and having a length in x-direction and a width in y-direction (which is the direction perpendicular to the paper plane of FIG. 1) being both several times larger than the thickness.

[0068] The mounting device 100 comprises an electrically conductive structure 102 which is here configured as two parallel aligned copper foils. Copper has a linear coefficient of thermal expansion of approximately 16.Math.10.sup.−6 K.sup.−1 as a first value of thermal expansion in all three orthogonal spatial directions x, y and z.

[0069] Moreover, the mounting device 100 comprises an electrically insulating structure 104 composed of two parallel oriented layers of FR4 material, i.e. glass fibers in a resin matrix. The electrically insulating structure 100 has a second value of thermal expansion in both the x-direction and the y-direction which is smaller than the first value. In x-direction, this value is approximately 14.Math.10.sup.−6 K.sup.−1, and in y-direction this value is approximately 12.Math.10.sup.−6 K.sup.−1. In view of the anisotropic properties of FR4 material, the coefficient of thermal expansion of the FR4 material is approximately 70.Math.10.sup.−6 K.sup.−1 in z-direction. Each of the electrically insulating layers of the electrically insulating structure 104 is located directly on a corresponding exposed main surface of a respective one of the copper foils.

[0070] A thermal expansion adjustment structure 106, here advantageously made of diamond like carbon (DLC), is positioned embedded within the electrically conductive structure 102, i.e. between the two copper foils, and has a third value of thermal expansion in all three spatial dimensions x, y, and z, i.e. an isotropic linear coefficient of thermal expansion of for instance about 2-3.Math.10.sup.−6 K.sup.−1 (wherein the exact value depends on the exact composition of the DLC, which can be adjusted as illustrated in FIG. 4). Since DLC has a high thermal conductivity and is electrically insulating, it contributes to the removal of heat during operation of the mounting device 100 and nevertheless does not disturb the transmission of electric signals in view of its dielectric properties.

[0071] This third value is hence selected to be smaller than both the first value and the second value, and the thermal expansion adjustment structure 106 is positioned symmetrically within the electrically conductive structure 102 so that thermally induced warpage of the mounting device 100 resulting from a difference between the first value and the second value is at least partially compensated by the thermal expansion adjustment structure 106. Thus, the thermal expansion adjustment structure 106 with its small CTE and its central location within the symmetric layer stack of FIG. 1 serves as a stiffener and compensates at least partially the different thermal expansion characteristics of copper on the one hand and FR4 on the other hand in the xy-plane. In particular, a composite formed by the thermal expansion adjustment structure 106 in combination with the electrically conductive structure 102 has substantially the same thermal expansion properties in the xy-plane as the electrically insulating structure 104. Therefore, undesired twist and bow phenomena of the plate shaped mounting device 100 are substantially prevented thanks to the presence of the thermal expansion adjustment structure 106.

[0072] In order to improve adhesion of the thermal expansion adjustment structure 106 on the electrically conductive structure 102, the mounting device 100 further comprises two layers constituting an adhesion promoting structure 108 between the thermal expansion adjustment structure 106 and the electrically conductive structure 102 for promoting adhesion of the thermal expansion adjustment structure 106 on the electrically conductive structure 102. In the shown embodiment, the adhesion promoting structure 108 comprises a carbide builder. Each of the two separate layers of the adhesion promoting structure 108 is sandwiched respectively between the central thermal expansion adjustment structure 106 and a respective one of the two copper foils constituting the electrically conductive structure 102. Thus, the layers 106, 108, 102, 104 form an axially symmetric structure.

[0073] The mounting device 100 furthermore comprises patterned copper layers on each of the opposing layers of the electrically insulating structure 104, the patterned copper layers constituting a further electrically conductive structure 110.

[0074] Interfaces 150, 160 between mutually contacting main surfaces of the electrically conductive structure 102 and the electrically insulating structure 104 are positions at which, additionally or alternatively to the shown thermal expansion adjustment structure 106, one or more thermal expansion adjustment structures with corresponding properties may be foreseen. By providing plural thermal expansion adjustment structures 106 in one mounting device 100, it is possible to further refine adjustment of the thermal expansion (and related mechanical load) properties of the mounting device 100.

[0075] FIG. 2 illustrates a cross sectional view of a mounting device 100 according to another exemplary embodiment of the invention. According to the mounting device 100 shown in FIG. 2, further patterned/continuous structures of electrically conductive material/electrically insulating material are provided in addition to the mounting device 100 shown in FIG. 1.

[0076] FIG. 3 illustrates a cross sectional view of a mounting device 100 according to yet another exemplary embodiment of the invention.

[0077] In contrast to the embodiments of FIG. 1 and FIG. 2, a layer-shaped thermal expansion adjustment structure 106 according to FIG. 3 contacts on one side the electrically insulating structure 104 and is separated on the opposing other side from a layer-shaped electrically conductive structure 102 by a layer-shaped adhesion promoting structure 108.

[0078] Furthermore, the electrically conductive structure 102 comprises cylindrical post-shaped vias electrically connecting further electrically conductive structures 110 in the form of patterned copper layers on both opposing main surfaces of the mounting device 100 to one another. The vias are separated from electrically insulating structure 104 by a respective hollow cylindrical thermal expansion adjustment structure 106 in direct contact with the electrically insulating structure 104 and by a respective hollow cylindrical adhesion promoting structure 108 in direct contact with the hollow cylindrical thermal expansion adjustment structure 106 and the respective post-shaped via. Thus, both thermal expansion adjustment and increase of the thermal conductivity may be accomplished at multiple positions within the mounting device 100 to thereby obtain a multi position warpage suppression and promotion of heat removal.

[0079] FIG. 4 illustrates a phase diagram 400 indicating contributions of sp.sup.2 hybridized carbon, sp.sup.3 hybridized carbon and hydrogen of a carbon comprising thermal expansion adjustment structure 106 of a mounting device 100 according to an exemplary embodiment of the invention, wherein mechanical and thermal properties of the mounting device 100 may be adjusted by configuring a manufacturing procedure in accordance with a desired section of the phase diagram 400.

[0080] According to the phase diagram 400, the thermal expansion adjustment structure 106 of diamond like carbon (DLC) is a hydrogen (H) comprising amorphous carbon coating with a mixture of sp.sup.2 and sp.sup.3 hybridized carbon. Preferably, the portion of sp.sup.2 hybridized carbon is in a range between 40 and 60 weight percent of the thermal expansion adjustment structure 106, the portion of sp.sup.3 hybridized carbon is in a range between 25 and 40 weight percent of the thermal expansion adjustment structure 106, and the percentage of hydrogen is above 10 weight percent (preferably not above 40 weight percent). When the thermally conductive and electrically insulating thermal expansion adjustment structure 106 is formed by sputtering/physical vapor deposition (PVD), the percentage of sp.sup.2 hybridized carbon is high. When however plasma enhanced chemical vapor deposition (PECVD) is used for forming the thermal expansion adjustment structure 106, a higher hydrogen percentage is obtained. With a high percentage of sp.sup.2 hybridized and sp.sup.3 hybridized carbon, a high thermal conductivity of the thermal expansion adjustment structure 106 may be obtained, and an appropriate coefficient of thermal expansion may be fine-tuned. With a high hydrogen percentage, a mechanically stable thermal expansion adjustment structure 106 is obtained. By a selection of the manufacturing procedure (for instance also adjustment of the precise process parameters and/or, if desired, a combination of the above-mentioned manufacturing procedures), the mechanical and thermal properties of the thermal expansion adjustment structure 106 may be precisely set. A particularly appropriate composition in terms of the mechanical and thermal properties is shown in FIG. 4 with an area denoted with reference numeral 402.

[0081] In an exemplary embodiment of the invention in which the thermal expansion adjustment structure is formed of diamond like carbon, a mixture of sp.sup.2 and sp.sup.3 hybridized carbon of the thermal expansion adjustment structure 106 is adjusted so (in particular using the phase diagram 400 of FIG. 4) that the thermally induced warpage of the mounting device 100 resulting from the difference between the above mentioned first value and the second value is at least partially compensated by the thermal expansion adjustment structure 106.

[0082] FIG. 5 illustrates a three-dimensional view of a mounting device 100 according to an exemplary embodiment of the invention having a bow-type warpage of a still acceptable extent and lying on a flat support 500.

[0083] In the event of a purely bow type warpage, all four corners of the rectangular mounting device 100 are placed at the same level, i.e. are in contact with the planar support 500.

[0084] In an embodiment, the thermal expansion adjustment structure 106 is configured so that the thermally induced warpage is characterized by a bow of the mounting device 100 of at the maximum 1.5% (in particular at the maximum 0.7%). The bow value is calculated as a ratio between a maximum distance, s, between a bottom surface of the mounting surface 100 and a planar support 500 carrying the mounting device 100 on the one hand and a length, D, of a longest side of the mounting device 100 on the other hand. In other words, the bow value is calculated as s/D.

[0085] FIG. 6 illustrates a three-dimensional view of a mounting device 100 according to an exemplary embodiment of the invention having a twist-type warpage of a still acceptable extent and lying on a flat support 500.

[0086] In the event of a purely twist type warpage, only three corners of the rectangular mounting device 100 are placed at the same level, i.e. are in contact with the planar support 500, whereas the fourth corner is spaced with regard to the flat surface of the support 500.

[0087] In an embodiment, the thermal expansion adjustment structure 106 is configured so that the thermally induced warpage is characterized by a twist of the mounting device 100 of at the maximum 2% (in particular at the maximum 1%). The twist is calculated as a ratio between a maximum distance, d, between the above-mentioned fourth corner of the mounting surface 100 and a planar support 500 carrying the mounting device 100 on the one hand and a longest diameter, L, of the mounting device 100 on the other hand. In other words, the twist value is calculated as d/L.

[0088] It has turned out that the above mentioned values of bow and twist, even when taken in combination, still provide an acceptable quality of the mounting device 100. The thermal expansion adjustment structure 106 may hence be configured so as to reduce the twist and bow values to fall below the given tolerable threshold values to obtain a mounting device 100 of sufficient quality.

[0089] It should be noted that the term “comprising” does not exclude other elements or steps and the “a” or “an” does not exclude a plurality. Also elements described in association with different embodiments may be combined.

[0090] It should also be noted that reference signs in the claims shall not be construed as limiting the scope of the claims.

[0091] Implementation of the invention is not limited to the preferred embodiments shown in the figures and described above. Instead, a multiplicity of variants are possible which use the solutions shown and the principle according to the invention even in the case of fundamentally different embodiments.