JP3795946B2

JP3795946B2 - Wiring board

Info

Publication number: JP3795946B2
Application number: JP33511795A
Authority: JP
Inventors: 彰一仲川
Original assignee: Kyocera Corp; Denso Corp
Current assignee: Kyocera Corp; Denso Corp
Priority date: 1995-12-22
Filing date: 1995-12-22
Publication date: 2006-07-12
Anticipated expiration: 2015-12-22
Also published as: JPH09181457A

Description

【０００１】
【発明の属する技術分野】
本発明は、半導体素子が収容搭載される半導体素子収納用パッケージや、半導体素子の他に抵抗体やコンデンサ等の各種電子部品が搭載される混成集積回路装置等に好適な配線基板に関するものである。
【０００２】
【従来の技術】
従来、半導体素子や抵抗体等に代表されるパワー素子を搭載する配線基板は、該パワー素子で発生した熱を放熱するために、モリブデン（Ｍｏ）や銅（Ｃｕ）等の金属製のヒートシンクが前記配線基板に形成された厚膜導体上に半田等を介して接合され、その上にパワー素子が搭載されていた。
【０００３】
しかしながら、前記構造では、過渡的な熱抵抗を下げるためにはヒートシンクを大型化する必要があり、実装体積が増加するとともにコストアップを招き、高密度、小型化の要求に反するものであった。
【０００４】
また、定常的な熱抵抗を低減するためにはヒートシンクの厚さを薄くすれば定常的な熱抵抗は低減できるものの、前記過渡的な熱抵抗の低減と相反する結果となる等の欠陥があった。
【０００５】
そこで係る問題を解消せんとして、半導体素子等で発生した熱を、図５に示す様に半導体素子８を搭載する直下の領域の絶縁層９に配置された熱伝達用導体１０を通して放熱する多層基板１１が提案されている（特開平７−１６２１５７号公報参照）。
【０００６】
【発明が解決しようとする課題】
しかしながら、前記多層基板１１では、熱伝達用導体１０の体積を大きくすることにより過渡的な熱抵抗を低減できることからヒートシンク（不図示）を併設しても該ヒートシンクを薄くして定常的な熱抵抗も下げることが可能となるものの、前記熱伝達用導体１０は高融点金属とセラミックスとの混合物であり、従来の金属製ヒートシンクを使用した場合に相当する熱抵抗値を達成して同等の効果を得るためには、熱伝達用導体１０の熱伝導率は多層基板１１の熱伝導率の約２．５倍以上が必要となる。
【０００７】
その結果、金属製ヒートシンクを用いた場合よりも熱抵抗を更に低減し熱放散性を高めるためには、熱伝導率の観点からセラミックスの添加量は２０重量％以下でなければならないが、一方で熱伝達用導体１０と多層基板１１間の熱膨張率差が生じることから不要な熱応力が発生し、多層基板１１に割れを生じたりするため、一つの目安として熱伝達用導体１０と多層基板１１との熱膨張率差に対する多層基板１１の熱膨張率の比率を約０．２以下にしなければならず、そうするためには前記セラミックスの含有量を２０重量％よりも多くする必要がある。
【０００８】
その結果、熱伝導率の観点からは熱伝達用導体１０の熱放散性を低下させることになるという課題があった。
【０００９】
更に、多層基板１１を熱伝達用導体１０とともに焼成する際、前記熱膨張率差に起因する熱応力が発生して多層基板１１内に残留し、特に熱伝達用導体１０のコーナー部近傍に集中して高応力となり、係る多層基板１１に外力や熱衝撃力が印加されると、容易に前記多層基板１１にクラックを発生させて配線導体１２を切断するなど信頼性に欠けるという課題があった。
【００１０】
【発明の目的】
本発明は前記課題に鑑み成されたもので、その目的は配線基板に搭載したパワー素子から発生される熱を速やかに放熱するとともに、配線導体の断線等を招く配線基板のクラック発生を有効に防止し、信頼性の高い配線基板を提供することにある。
【００１１】
【課題を解決するための手段】
本発明の配線基板は、半導体素子や抵抗体等のパワー素子が搭載される複数の絶縁層から成る配線基板の前記半導体素子直下の下部領域に高融点金属とセラミックスから成る略矩形状の熱伝達層を配設したものであって、前記絶縁層と同時焼成され、５〜２０重量％のセラミックスを含有し、かつコーナー部が前記略矩形状の短辺の１／１０〜１／２の範囲内の曲率を有する熱伝達層を配設したことを特徴とするものである。また、本発明の配線基板は、前記熱伝達層が絶縁層に形成された凹部に配設されるとともに、前記凹部が前記熱伝達層のコーナー部の曲率に対応した曲率を有することを特徴とするものである。また、本発明の配線基板は、前記熱伝導層の面積が、前記パワー素子の面積よりも大きいことを特徴とするものである。
【００１２】
【作用】
本発明の配線基板は、搭載される半導体素子や抵抗体等のパワー素子の下部領域に配設した略矩形状の熱伝達層が、高融点金属と５〜２０重量％のセラミックスを含有し、前記絶縁層と同時焼成され、かつそのコーナー部が前記略矩形状の短辺の１／１０〜１／２の範囲内の曲率を有するものであることから、前記熱伝達層と配線基板の両者の熱膨張率差に起因して発生する熱応力が、熱伝達層のコーナー部近傍に集中するのが緩和され、その結果、配線基板に外力や熱衝撃力が印加されても配線基板にクラック等の不具合を生じることがなく、配線導体の断線等も有効に防止することができ、前記熱伝達層を配線基板に強固に取着させることが可能となり、熱伝達層と配線基板間の熱伝導特性も高く維持される。
【００１３】
【発明の実施の形態】
以下、本発明の配線基板の一実施例を図面に基づき詳述する。
図１は、本発明をアルミナ（Ａｌ₂Ｏ₃）を主成分とする３層構成の絶縁層と熱伝達層から成る配線基板に適用した斜視図であり、図２及び図３は図１の熱伝達層のコーナー部の一部を示す拡大図であり、図２は単一の曲率を有するコーナー部の一部を示す拡大図であり、図３は複数の曲率で構成されるコーナー部の一部を示す拡大図である。
【００１４】
図１乃至図３において、１はアルミナ（Ａｌ₂Ｏ₃）を主成分とする３層構成の絶縁層２と、表層から第２層目までの厚さを有し、そのコーナー部４が所定範囲内の曲率５を有して成る熱伝達層３とから成る配線基板であり、熱伝達層３の上面には半導体素子７が搭載されることになる。
【００１５】
即ち、前記３層構成の絶縁層２の表層から第２層の所定領域には、コーナー部４が所定の曲率５を有する平面形状が矩形状の凹部が形成され、前記曲率５は矩形状の短辺６の長さの１／１０〜１／２の範囲内の任意の値を有するものであり、この凹部に高融点金属とセラミックスから成る熱伝導性の良好な混合物が充填され、焼成一体化されて熱伝達層３が形成されている。
【００１６】
本発明における熱伝達層３を構成するセラミックスの含有量が、５重量％未満になると前記配線基板１と熱伝達層３との間に生じる両者の熱膨張率差に起因する熱応力が大となり、前記コーナー部４を設けただけでは効果的に低減することが不可能となり、また、２０重量％を越えると熱伝達層３の熱伝導率が低下し、従来の金属製ヒートシンクを用いた場合の熱抵抗より大となり、熱放散性が劣ることから、その含有量は５〜２０重量％の範囲となり、特に熱放散性と熱応力を勘案するとその含有量は１０〜１５重量％が最も好ましい。また、この熱伝達層３の面積はパワー素子７の面積よりも大きくすることが好ましい。
【００１７】
また、前記熱伝達層３を構成する高融点金属は、１０００〜１６００℃の温度範囲で積層したセラミックグリーンシートと同時焼成が可能なものであればいずれでも良く、特にモリブデン（Ｍｏ）及びタングステン（Ｗ）が好適である。
【００１８】
更に、前記熱伝達層３を構成するセラミックスとしては、前記高融点金属と同時焼成可能なものであればいずれでも良いが、特に絶縁性に優れたアルミナ（Ａｌ₂Ｏ₃）をはじめとする酸化物系セラミックスや、熱放散性に優れた窒化珪素（Ｓｉ₃Ｎ₄）や窒化アルミニウム（ＡｌＮ）等の窒化物系セラミックスが好適である。
【００１９】
一方、前記熱伝達層３のコーナー部４の曲率５が、略矩形状を成す熱伝達層３の短辺６の長さの１／１０未満の場合、熱衝撃等の外力が加わると容易にクラックが発生し、抵抗変化が認められ、１／２を越えると応力集中を回避することができず、焼成により配線基板１にクラックを生じてしまうことから、前記曲率５は１／１０〜１／２の範囲内に特定され、特に実装面積を小さくすること、及び熱伝達層３の体積を極小化して低コスト化を図るという点からは１／６〜１／４が好適である。
【００２０】
尚、前記熱伝達層３の厚さは、配線基板１の収縮率及び熱膨張率との相違に起因する反りや熱放散性の点からは、配線基板１の厚さに近い程好ましく、配線基板１と同一厚さで、裏面にまで突き抜けているのが最良となる。
【００２１】
【実施例】
本発明の配線基板を評価するに際し、先ず、アルミナ質焼結体からなる絶縁基体として、例えば、アルミナ（Ａｌ₂Ｏ₃）、シリカ（ＳｉＯ₂）、マグネシア（ＭｇＯ）、カルシア（ＣａＯ）等の原料粉末に公知の有機バインダー、可塑剤、溶剤を添加混合して泥漿を調製し、該泥漿を周知のドクターブレード法、カレンダーロール法等のテープ成形技術により厚さ約３００μｍのセラミックグリーンシートを成形した後、該セラミックグリーンシートの所定位置に予め打ち抜き加工を施して熱伝達層形成用の空所を形成した。
【００２２】
その後、タングステン（Ｗ）、モリブデン（Ｍｏ）等の高融点金属を主たる成分とする粉末に、アルミナ粉末を表１に示す組成で添加し、公知の有機バインダー、可塑剤、溶剤を添加混合して得たペーストを用いて前記セラミックグリーンシート上に所定の配線パターンを印刷塗布するとともに、前記熱伝達層形成用の空所とスルーホール部等にスクリーン印刷あるいは圧力充填法により前記ペーストを充填した。
【００２３】
また、通電試験を行うために、表層を除く各層に相当するセラミックグリーンシートにも所定の配線パターンをスクリーン印刷した。
【００２４】
次いで、前記各グリーンシートを積層し、これを水素（Ｈ₂）と窒素（Ｎ₂）等の混合ガスから成る還元性雰囲気中、約１６００℃の温度で焼成することにより、厚さ約２５０μｍまたは約５００μｍの熱伝達層を内蔵した３層からなる評価用の配線基板を得た。
【００２５】
尚、前記熱伝達層の厚さは、該熱伝達層を含む断面で前記評価用の配線基板を切断し、該断面をマイクロメータ付き顕微鏡を用いて測定した。
【００２６】
また、前記熱伝達層のコーナー部の曲率は、前記評価用の配線基板を熱伝達層が露出している上面から、マイクロメータ付き顕微鏡により熱伝達層の短辺の寸法と曲率を測定し、該短辺の寸法に対する曲率の比率から求めた。
【００２７】
尚、多層のアルミナ質焼結体から成る厚さが約７５０μｍの配線基板に、厚さが約５００μｍで、一辺の長さが約４ｍｍの正方形を底面とする板状のモリブデン製ヒートシンクを接合した評価用の配線基板を、従来技術の金属製ヒートシンクを設けた配線基板とし、評価基準とした。
一方、熱伝達層を高融点金属のみで構成したものを比較例とした。
【００２８】
【表１】

【００２９】
かくして得られた評価用の配線基板を用いて、熱伝達層が露出している面及びその裏面にレッドチェック液等の浸透探傷液で処理した後、顕微鏡で目視検査を行い、熱伝達層を取り巻く配線基板の絶縁部のクラックの有無を確認した。
【００３０】
また、該評価用の配線基板に高温設定温度１５０℃、低温設定温度−４０℃の熱衝撃を６サイクル加える熱衝撃試験を施し、前期同様の浸透探傷液で処理して顕微鏡で目視検査を行い残留する熱応力と熱衝撃の効果により進行した配線基板の絶縁部表面のクラックの有無を確認した。
【００３１】
一方、前記配線基板内部の配線導体の抵抗を熱衝撃試験前後で、抵抗測定器を用いて４端子法により測定し、前述した残留応力と熱衝撃の効果により進行した配線基板の絶縁部のクラックによる配線導体の断線の有無を、前記試験前後の抵抗変化の有無により確認した。
【００３２】
また、熱伝導率比は、配線基板より絶縁部及び熱伝達層部を切り出して測定試料を作製し、レーザーフラッシュ法にて前記絶縁部及び熱伝達層部の熱伝導率を個々に測定して算出した。
【００３３】
次に、配線基板と熱伝達層との熱膨張率比は、配線基板より絶縁部と熱伝達層部を切り出して測定試料を作製し、熱膨張測定装置（ＴＭＡ）で両部を個別に測定し、その差を絶縁部の熱膨張率で除して算出した。
【００３４】
【表２】

【００３５】
表から明らかなように、従来のヒートシンクを設置した試料番号１では熱伝導率比が２．５と低く、本発明の請求範囲外の試料番号２、３、１７では焼成後の絶縁層に既にクラックが認められ、試料番号４、９は熱衝撃試験後にクラックが認められたもので、試料番号１６は熱放散性が劣り、熱抵抗が大であるというものである。
【００３６】
それらに対して本発明の試料は、いずれもクラックは皆無であり、配線導体の断線は認められなかった。
【００３７】
また、表の結果に基づき、コーナー部の曲率を１／６とした場合の熱伝達層のアルミナ添加量と、配線基板と熱伝達層との熱膨張率差に対する配線基板の熱膨張率の比率の関係と配線基板のクラックの有無を図４に図示する。
【００３８】
図４から明らかなように、アルミナ添加量は５重量％以上で２０重量％以下の範囲内では比率が０．３以下となり、配線基板にはクラックが発生していないことが分かる。
【００３９】
また、有限要素法を用いたシミュレーションを実施したところ、本発明に係る熱伝達層のコーナー部に所定の曲率を有する配線基板は、いずれも熱伝達層と絶縁層との間に発生する両者の熱膨張差に起因する熱応力の前記熱伝達層のコーナー部近傍の絶縁層への応力集中が緩和されていることが確認された。
【００４０】
【発明の効果】
本発明の配線基板によれば、配線基板に熱衝撃等の外部応力が印加されても配線導体の断線等を招く絶縁層のクラック発生を有効に防止するとともに、熱伝達層を絶縁層と強固に取着させておくことができ、熱伝達層と絶縁層との熱伝導特性も良好に維持でき、配線基板に搭載した半導体素子や抵抗体等に代表されるパワー素子から発生される熱を速やかに放熱することができる信頼性の高い配線基板が得られる。
【図面の簡単な説明】
【図１】本発明をアルミナ（Ａｌ₂Ｏ₃）を主成分とする３層構成の絶縁層と熱伝達層から成る配線基板に適用した斜視図である。
【図２】本発明の配線基板の熱伝達層のコーナー部が、単一の曲率で構成されたことをしめす部分拡大図である。
【図３】本発明の配線基板の熱伝達層のコーナー部が、複数の曲率で構成されたことをしめす部分拡大図である。
【図４】コーナー部の曲率を熱伝達層の短片の１／６とした場合の熱伝達層のアルミナ添加量と、配線基板と熱伝達層との熱膨張率差に対する配線基板の熱膨張率の比率の関係と配線基板のクラックの有無を示す図である。
【図５】従来の多層基板の要部を示す断面図である。
【符号の説明】
１配線基板
２絶縁層
３熱伝達層
４コーナー部
５曲率
６短辺[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a wiring board suitable for a semiconductor element housing package in which a semiconductor element is accommodated and a hybrid integrated circuit device in which various electronic components such as a resistor and a capacitor are mounted in addition to the semiconductor element. .
[0002]
[Prior art]
Conventionally, a wiring board on which a power element typified by a semiconductor element or a resistor is mounted has a heat sink made of metal such as molybdenum (Mo) or copper (Cu) in order to dissipate heat generated by the power element. The power element is mounted on the thick film conductor formed on the wiring board by soldering or the like.
[0003]
However, in the above structure, it is necessary to increase the size of the heat sink in order to lower the transient thermal resistance, which increases the mounting volume and increases the cost, which is against the demand for high density and downsizing.
[0004]
Further, in order to reduce the steady thermal resistance, the steady thermal resistance can be reduced by reducing the thickness of the heat sink, but there are defects such as a result contrary to the transient thermal resistance reduction. It was.
[0005]
To solve this problem, a multilayer board that radiates heat generated in a semiconductor element or the like through a heat transfer conductor 10 disposed in an insulating layer 9 in a region immediately below where the semiconductor element 8 is mounted as shown in FIG. 11 has been proposed (see Japanese Patent Application Laid-Open No. 7-162157).
[0006]
[Problems to be solved by the invention]
However, in the multilayer substrate 11, since the transient thermal resistance can be reduced by increasing the volume of the heat transfer conductor 10, even if a heat sink (not shown) is additionally provided, the heat sink is thinned to provide a steady thermal resistance. However, the heat transfer conductor 10 is a mixture of a refractory metal and a ceramic, and achieves a thermal resistance value equivalent to that obtained when a conventional metal heat sink is used. In order to obtain this, the heat conductivity of the heat transfer conductor 10 needs to be about 2.5 times or more that of the multilayer substrate 11.
[0007]
As a result, in order to further reduce the thermal resistance and increase the heat dissipation than when using a metal heat sink, the amount of ceramics added must be 20% by weight or less from the viewpoint of thermal conductivity. Since a difference in coefficient of thermal expansion between the heat transfer conductor 10 and the multilayer substrate 11 is generated, unnecessary thermal stress is generated and the multilayer substrate 11 is cracked. Therefore, as one guideline, the heat transfer conductor 10 and the multilayer substrate are used. The ratio of the thermal expansion coefficient of the multilayer substrate 11 to the difference in thermal expansion coefficient with respect to 11 must be about 0.2 or less, and in order to do so, the ceramic content needs to be more than 20% by weight. .
[0008]
As a result, there has been a problem in that the heat dissipating property of the heat transfer conductor 10 is reduced from the viewpoint of thermal conductivity.
[0009]
Further, when the multilayer substrate 11 is baked together with the heat transfer conductor 10, thermal stress due to the difference in thermal expansion coefficient is generated and remains in the multilayer substrate 11, particularly concentrated near the corner portion of the heat transfer conductor 10. When an external force or a thermal shock force is applied to the multilayer substrate 11, the multilayer conductor 11 easily cracks and the wiring conductor 12 is cut, resulting in lack of reliability. .
[0010]
OBJECT OF THE INVENTION
The present invention has been made in view of the above problems, and its purpose is to quickly dissipate the heat generated from the power element mounted on the wiring board, and to effectively generate cracks in the wiring board that lead to disconnection of the wiring conductor. An object of the present invention is to provide a highly reliable wiring board.
[0011]
[Means for Solving the Problems]
The wiring board of the present invention has a substantially rectangular heat transfer made of a refractory metal and ceramics in a lower region immediately below the semiconductor element of the wiring board made of a plurality of insulating layers on which power elements such as semiconductor elements and resistors are mounted. A layer is provided, which is co-fired with the insulating layer , contains 5 to 20% by weight of ceramics, and has a corner portion in a range of 1/10 to 1/2 of the substantially rectangular short side A heat transfer layer having an inner curvature is provided. The wiring board of the present invention is characterized in that the heat transfer layer is disposed in a recess formed in an insulating layer, and the recess has a curvature corresponding to a curvature of a corner portion of the heat transfer layer. To do. The wiring board of the present invention is characterized in that the area of the heat conductive layer is larger than the area of the power element.
[0012]
[Action]
In the wiring board of the present invention, a substantially rectangular heat transfer layer disposed in a lower region of a power element such as a semiconductor element or a resistor to be mounted contains a refractory metal and 5 to 20% by weight of ceramic, Since both the heat transfer layer and the wiring substrate are fired at the same time as the insulating layer and the corner portion has a curvature within a range of 1/10 to 1/2 of the substantially rectangular short side. The thermal stress generated due to the difference in thermal expansion coefficient of the heat transfer layer is alleviated from concentrating near the corner of the heat transfer layer, and as a result, even if an external force or thermal shock force is applied to the wiring board, the wiring board is cracked. In this way, disconnection of the wiring conductor can be effectively prevented, the heat transfer layer can be firmly attached to the wiring board, and heat between the heat transfer layer and the wiring board can be obtained. Conductivity characteristics are also maintained high.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an embodiment of a wiring board according to the present invention will be described in detail with reference to the drawings.
FIG. 1 is a perspective view in which the present invention is applied to a wiring board composed of a three-layered insulating layer mainly composed of alumina (Al ₂ O ₃ ) and a heat transfer layer, and FIGS. FIG. 2 is an enlarged view showing a part of the corner portion of the heat transfer layer, FIG. 2 is an enlarged view showing a part of the corner portion having a single curvature, and FIG. 3 is a view of the corner portion constituted by a plurality of curvatures. It is an enlarged view which shows a part.
[0014]
1 to 3, reference numeral 1 denotes an insulating layer 2 having a three-layer structure mainly composed of alumina (Al ₂ O ₃ ) and a thickness from the surface layer to the second layer. A wiring board comprising a heat transfer layer 3 having a curvature 5 within the range, and a semiconductor element 7 is mounted on the upper surface of the heat transfer layer 3.
[0015]
That is, in a predetermined region from the surface layer to the second layer of the insulating layer 2 having the three-layer structure, a concave portion having a rectangular shape with a corner portion 4 having a predetermined curvature 5 is formed, and the curvature 5 has a rectangular shape. It has an arbitrary value within the range of 1/10 to 1/2 of the length of the short side 6, and this concave portion is filled with a mixture having a high thermal conductivity made of a refractory metal and ceramics, and fired integrally. Thus, the heat transfer layer 3 is formed.
[0016]
If the content of the ceramics constituting the heat transfer layer 3 in the present invention is less than 5% by weight, the thermal stress due to the difference in thermal expansion coefficient between the wiring board 1 and the heat transfer layer 3 becomes large. When the corner portion 4 is provided, it is impossible to effectively reduce the temperature. When the amount exceeds 20% by weight, the thermal conductivity of the heat transfer layer 3 is lowered, and a conventional metal heat sink is used. Therefore, the content is in the range of 5 to 20% by weight, and the content is most preferably 10 to 15% by weight in consideration of heat dissipation and thermal stress. . The area of the heat transfer layer 3 is preferably larger than the area of the power element 7.
[0017]
The refractory metal constituting the heat transfer layer 3 may be any material as long as it can be co-fired with a ceramic green sheet laminated in a temperature range of 1000 to 1600 ° C., in particular molybdenum (Mo) and tungsten ( W) is preferred.
[0018]
Further, the ceramic constituting the heat transfer layer 3 may be any ceramic as long as it can be co-fired with the refractory metal, but is particularly oxidized such as alumina (Al ₂ O ₃ ) having excellent insulating properties. A material ceramic, or a nitride ceramic such as silicon nitride (Si ₃ N ₄ ) or aluminum nitride (AlN) excellent in heat dissipation is suitable.
[0019]
On the other hand, when the curvature 5 of the corner portion 4 of the heat transfer layer 3 is less than 1/10 of the length of the short side 6 of the heat transfer layer 3 having a substantially rectangular shape, it is easy to apply an external force such as thermal shock. Cracks are generated and resistance change is observed. If the ratio exceeds 1/2, stress concentration cannot be avoided, and the wiring substrate 1 is cracked by firing. In particular, 1/6 to 1/4 is preferable from the viewpoint of reducing the mounting area and minimizing the volume of the heat transfer layer 3 to reduce the cost.
[0020]
The thickness of the heat transfer layer 3 is preferably closer to the thickness of the wiring board 1 in terms of warpage and heat dissipation due to the difference between the shrinkage rate and the thermal expansion coefficient of the wiring board 1. It is best to have the same thickness as the substrate 1 and penetrate through to the back surface.
[0021]
【Example】
When evaluating the wiring board of the present invention, first, as an insulating base made of an alumina sintered body, for example, alumina (Al ₂ O ₃ ), silica (SiO ₂ ), magnesia (MgO), calcia (CaO), etc. A known organic binder, plasticizer, and solvent are added to the raw material powder to prepare a slurry, and the slurry is formed into a ceramic green sheet having a thickness of about 300 μm by a tape forming technique such as a known doctor blade method or calendar roll method. After that, a blanking space for forming a heat transfer layer was formed by punching the ceramic green sheet at a predetermined position in advance.
[0022]
Thereafter, alumina powder is added to the powder mainly composed of a high melting point metal such as tungsten (W), molybdenum (Mo), etc. with the composition shown in Table 1, and a known organic binder, plasticizer and solvent are added and mixed. A predetermined wiring pattern was printed and applied on the ceramic green sheet using the obtained paste, and the paste was filled into the voids and through-hole portions for forming the heat transfer layer by screen printing or pressure filling.
[0023]
Further, in order to conduct an energization test, a predetermined wiring pattern was screen printed on a ceramic green sheet corresponding to each layer except the surface layer.
[0024]
Next, the green sheets are laminated and fired at a temperature of about 1600 ° C. in a reducing atmosphere composed of a mixed gas such as hydrogen (H ₂ ) and nitrogen (N ₂ ), so that the thickness is about 250 μm or A wiring board for evaluation consisting of three layers with a built-in heat transfer layer of about 500 μm was obtained.
[0025]
The thickness of the heat transfer layer was measured by cutting the wiring board for evaluation at a cross section including the heat transfer layer, and using a microscope with a micrometer.
[0026]
Further, the curvature of the corner portion of the heat transfer layer is measured from the upper surface where the heat transfer layer is exposed on the wiring board for evaluation, using the microscope with a micrometer, the dimension and curvature of the short side of the heat transfer layer, It calculated | required from the ratio of the curvature with respect to the dimension of this short side.
[0027]
In addition, a plate-like molybdenum heat sink having a square bottom surface with a thickness of about 500 μm and a side length of about 4 mm was joined to a wiring board made of a multilayer alumina sintered body and having a thickness of about 750 μm. The wiring board for evaluation was a wiring board provided with a metal heat sink of the prior art and used as an evaluation standard.
On the other hand, a heat transfer layer composed of only a refractory metal was used as a comparative example.
[0028]
[Table 1]

[0029]
Using the wiring board for evaluation obtained in this way, the surface where the heat transfer layer is exposed and the back surface thereof are treated with a penetrant flaw detection liquid such as a red check liquid, then visually inspected with a microscope, and the heat transfer layer is removed. The presence or absence of cracks in the insulating portion of the surrounding wiring board was confirmed.
[0030]
In addition, a thermal shock test is applied to the circuit board for evaluation for six cycles of thermal shock at a high temperature set temperature of 150 ° C. and a low temperature set temperature of −40 ° C. The presence or absence of cracks on the surface of the insulating portion of the wiring board, which was advanced by the effects of residual thermal stress and thermal shock, was confirmed.
[0031]
On the other hand, the resistance of the wiring conductor inside the wiring board was measured by a four-terminal method using a resistance measuring instrument before and after the thermal shock test. The presence or absence of disconnection of the wiring conductor due to was confirmed by the presence or absence of a resistance change before and after the test.
[0032]
In addition, the thermal conductivity ratio is obtained by cutting the insulating part and the heat transfer layer part from the wiring board to prepare a measurement sample, and individually measuring the thermal conductivity of the insulating part and the heat transfer layer part by a laser flash method. Calculated.
[0033]
Next, the thermal expansion coefficient ratio between the wiring board and the heat transfer layer is measured by cutting the insulating part and the heat transfer layer part from the wiring board to produce a measurement sample, and separately measuring both parts with a thermal expansion measuring device (TMA). The difference was calculated by dividing the difference by the coefficient of thermal expansion of the insulating portion.
[0034]
[Table 2]

[0035]
As is apparent from the table, the thermal conductivity ratio is as low as 2.5 in the sample No. 1 in which the conventional heat sink is installed, and in the sample Nos. 2, 3, and 17 outside the scope of the present invention, the insulating layer after firing is already present. Cracks were observed, Sample Nos. 4 and 9 were cracked after the thermal shock test, and Sample No. 16 was inferior in heat dissipation and large in thermal resistance.
[0036]
In contrast, none of the samples of the present invention had any cracks, and no disconnection of the wiring conductor was observed.
[0037]
Also, based on the results in the table, the amount of alumina added to the heat transfer layer when the curvature of the corner is 1/6, and the ratio of the thermal expansion coefficient of the wiring board to the difference in thermal expansion coefficient between the wiring board and the heat transfer layer 4 and the presence or absence of cracks in the wiring board are shown in FIG.
[0038]
As can be seen from FIG. 4, when the amount of alumina added is in the range of 5 wt% or more and 20 wt% or less, the ratio is 0.3 or less, and it can be seen that no cracks are generated in the wiring board.
[0039]
Moreover, when a simulation using the finite element method was performed, both of the wiring boards having a predetermined curvature at the corner portion of the heat transfer layer according to the present invention were generated between the heat transfer layer and the insulating layer. It was confirmed that the stress concentration due to the thermal stress due to the thermal expansion difference on the insulating layer near the corner portion of the heat transfer layer was alleviated.
[0040]
【The invention's effect】
According to the wiring board of the present invention, even when an external stress such as a thermal shock is applied to the wiring board, cracks in the insulating layer that cause disconnection of the wiring conductor are effectively prevented, and the heat transfer layer is firmly connected to the insulating layer. The heat conduction characteristics between the heat transfer layer and the insulating layer can be maintained well, and the heat generated from the power elements represented by semiconductor elements and resistors mounted on the wiring board can be maintained. A highly reliable wiring board that can quickly dissipate heat is obtained.
[Brief description of the drawings]
FIG. 1 is a perspective view in which the present invention is applied to a wiring board comprising a three-layered insulating layer mainly composed of alumina (Al ₂ O ₃ ) and a heat transfer layer.
FIG. 2 is a partially enlarged view showing that the corner portion of the heat transfer layer of the wiring board of the present invention is configured with a single curvature.
FIG. 3 is a partially enlarged view showing that the corner portion of the heat transfer layer of the wiring board of the present invention is configured with a plurality of curvatures.
FIG. 4 shows the amount of alumina added to the heat transfer layer and the thermal expansion coefficient of the wiring board with respect to the difference in thermal expansion coefficient between the wiring board and the heat transfer layer when the curvature of the corner portion is 1/6 of the short piece of the heat transfer layer. It is a figure which shows the relationship of these ratios and the presence or absence of the crack of a wiring board.
FIG. 5 is a cross-sectional view showing a main part of a conventional multilayer substrate.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Wiring board 2 Insulation layer 3 Heat transfer layer 4 Corner part 5 Curvature 6 Short side

Claims

In the lower region power element of a wiring board comprising a plurality of insulating layers are mounted, Ri formed of a refractory metal and ceramics, there wiring board which is disposed a heat transfer layer of the insulating layer and cofired rectangular The heat transfer layer contains 5 to 20% by weight of ceramic, and the curvature of the corner portion of the heat transfer layer is 1/10 to 1/2 of the short side of the heat transfer layer. Wiring board.

The wiring according to claim 1, wherein the heat transfer layer is disposed in a recess formed in the insulating layer, and the recess has a curvature corresponding to a curvature of a corner portion of the heat transfer layer. substrate.

The wiring board according to claim 1, wherein an area of the heat conductive layer is larger than an area of the power element.