IT201900003511A1 - Metodo per la fabbricazione di dispositivi integrati includenti una piastrina fissata ad un supporto di connessione - Google Patents

Metodo per la fabbricazione di dispositivi integrati includenti una piastrina fissata ad un supporto di connessione Download PDF

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Publication number
IT201900003511A1
IT201900003511A1 IT102019000003511A IT201900003511A IT201900003511A1 IT 201900003511 A1 IT201900003511 A1 IT 201900003511A1 IT 102019000003511 A IT102019000003511 A IT 102019000003511A IT 201900003511 A IT201900003511 A IT 201900003511A IT 201900003511 A1 IT201900003511 A1 IT 201900003511A1
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IT
Italy
Prior art keywords
solder
copper
connection support
silver
alloy
Prior art date
Application number
IT102019000003511A
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English (en)
Inventor
Michele Calabretta
Crocifisso Marco Antonio Renna
Sebastiano Russo
Marco Alfio Torrisi
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St Microelectronics Srl
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Publication date
Application filed by St Microelectronics Srl filed Critical St Microelectronics Srl
Priority to IT102019000003511A priority Critical patent/IT201900003511A1/it
Priority to US16/811,964 priority patent/US11482503B2/en
Priority to CN202010163686.1A priority patent/CN111682008A/zh
Publication of IT201900003511A1 publication Critical patent/IT201900003511A1/it
Priority to US17/933,016 priority patent/US11756916B2/en

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    • H—ELECTRICITY
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    • H01L23/495—Lead-frames or other flat leads
    • H01L23/49579—Lead-frames or other flat leads characterised by the materials of the lead frames or layers thereon
    • H01L23/49582—Metallic layers on lead frames
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K35/00—Rods, electrodes, materials, or media, for use in soldering, welding, or cutting
    • B23K35/22—Rods, electrodes, materials, or media, for use in soldering, welding, or cutting characterised by the composition or nature of the material
    • B23K35/24—Selection of soldering or welding materials proper
    • B23K35/26—Selection of soldering or welding materials proper with the principal constituent melting at less than 400 degrees C
    • B23K35/262—Sn as the principal constituent
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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Description

DESCRIZIONE
del brevetto per invenzione industriale dal titolo:
“METODO PER LA FABBRICAZIONE DI DISPOSITIVI INTEGRATI INCLUDENTI UNA PIASTRINA FISSATA AD UN SUPPORTO DI CONNESSIONE”
La presente invenzione si riferisce ad un metodo di brasatura che avviene mediante la diffusione di una specie chimica all’interno di una lega di saldatura riscaldata oltre la sua temperatura di fusione. Tale tecnica è utilizzata per la fabbricazione di circuiti elettronici integrati, in particolare per la saldatura di una piastrina (″die″) con un supporto di connessione, tipicamente una leadframe.
E’ noto che durante la fabbricazione di dispositivi elettronici integrati, la piastrina, che integra componenti elettronici e/o altri componenti realizzati con la tecnologia dei semiconduttori, quali dispositivi di potenza, deve essere saldata ad un supporto di connessione (anche noto come “leadframe” o semplicemente “frame”). Tale processo di saldatura viene comunemente chiamato “die attach”.
Il frame svolge il ruolo di supporto meccanico per la piastrina, consente il contatto elettrico della piastrina con l'esterno e può servire per dissipare il calore generato all’interno del dispositivo.
Pertanto il materiale mediante il quale viene eseguita la saldatura deve avere bassa resistenza elettrica e alta conducibilità termica.
La saldatura deve inoltre resistere a stress ripetuti e mantenere la propria integrità. Tali stress sono dovuti alle variazioni di temperatura dei circuiti elettronici durante la loro vita applicativa. Poiché la piastrina e il frame hanno coefficienti di espansione termica differenti, essi tendono, al variare della temperatura operativa, ad espandersi in maniera differente l’un l’altro generando così, a causa del vincolo che li lega, degli stress interni alla struttura. Pertanto, il sistema costituito dalla piastrina, dal frame e dal materiale di saldatura subisce uno stress meccanico in funzione della temperatura.
Tra i diversi metodi per eseguire la saldatura metallurgica di una piastrina con un frame metallico si possono annoverare: la saldatura eseguita utilizzando leghe a base di oro (detta “hard die-attach”) e la brasatura dolce utilizzando leghe a base di piombo o stagno.
Nel primo caso si creano legami forti con eccellenti proprietà termiche ed elettriche, ma la rigidità può causare la frattura della piastrina durante i cicli termici in quanto la saldatura rigida trasferisce lo stress termico alla piastrina che, da materiale fragile, non può deformarsi plasticamente.
Inoltre, le leghe di saldatura a base di oro usate per la saldatura “hard die-attach”, come ad esempio la lega 80Au-20Sn, hanno costi molto elevati e non sono pertanto commercialmente vantaggiose. Infine, le procedure per la saldatura con tali leghe, che possono essere usate per realizzare processi di diffusion soldering (attraverso la diffusione di una specie chimica presente nella frame), è complessa e richiede fasi di processo e macchinari dedicati non comunemente utilizzati nella fabbricazione di circuiti elettronici.
La saldatura dolce con leghe a base di piombo, invece, è sufficientemente plastica per compensare le differenze nell’espansione termica tra la piastrina ed il frame, ma è comunque suscettibile a cedimento a fatica dopo ripetuti cicli termici.
Tuttavia, i sempre più stringenti requisiti per la limitazione dell’uso di sostanze pericolose nella fabbricazione di dispositivi elettronici hanno portato allo sviluppo di leghe per saldatura a basso contenuto o addirittura prive di piombo.
Ad esempio US4170472 illustra leghe a base di stagno, argento e antimonio, tra cui, particolarmente preferita è la lega 65Sn-25Ag-10Sb (nota anche come “J alloy”).
US4170472 descrive un metodo per la saldatura di una piastrina rivestita con cromo/argento o nichel/stagno con un frame rivestito con nichel o argento.
Tuttavia, tali saldature si sono dimostrate poco resistenti a ripetuti cicli termici, risultando in una scarsa affidabilità delle prestazioni del sistema.
Lo scopo della presente invenzione è quello di risolvere i problemi tecnici precedentemente menzionati.
In particolare, è scopo della presente invenzione quello di fornire un nuovo metodo per la saldatura di una piastrina con un supporto di connessione e un dispositivo integrato incorporante gli stessi.
Lo scopo della presente invenzione è raggiunto da un metodo e da un dispositivo secondo le rivendicazioni allegate.
In particolare, la descrizione che segue si riferisce ad un metodo per la saldatura di una piastrina con un supporto di connessione comprendente le fasi di:
a) fornire un supporto di connessione presentante almeno una superficie realizzata almeno parzialmente in rame;
b) fornire una piastrina presentante almeno una superficie rivestita con uno strato metallico;
c) applicare alla superficie realizzata almeno parzialmente in rame del supporto di connessione una lega di saldatura e scaldare la lega ad una temperatura di almeno 380°C per formare una goccia di lega di saldatura in cui la lega di saldatura è priva di piombo ed è scelta nel gruppo costituito da leghe comprendenti almeno il 40% in peso di stagno, leghe comprendenti almeno il 50% di indio e leghe comprendenti almeno il 50% di gallio;
d) porre a contatto lo strato metallico della piastrina con la goccia di lega di saldatura per formare la saldatura con il supporto di connessione.
Vantaggiosamente, il metodo di saldatura può essere eseguito su un supporto di connessione realizzato interamente in rame o alternativamente presentante solo una superficie rivestita almeno parzialmente di rame.
La lega utilizzata può essere una lega scelta nel gruppo costituito da:
- 61-69% stagno, 8-11% antimonio e 23-28% argento; - 50-99% stagno, 1-5% argento;
- 98-98,9% stagno, 1-5% argento, 0,1-1% rame; - 50-99% indio, 1-50% argento;
- 50-99% gallio, 1-50% nichel;
- 50-99% gallio, 1-50% rame; e
- 70-99% gallio, 1-30% argento;
le percentuali essendo espresse in peso sul peso totale della lega. Più in particolare la lega contiene 65% stagno, 25% argento, 10% antimonio (65Sn-25Ag-10Sb), le percentuali essendo espresse in peso sul peso totale della lega.
In una forma di realizzazione, la piastrina è provvista di almeno una superficie rivestita con uno strato metallico in particolare realizzato con un metallo selezionato nel gruppo costituito da titanio, nichel, oro, argento, alluminio e rame.
Vantaggiosamente, l’utilizzo di un frame in rame o rivestito anche solo parzialmente con rame e di una lega di saldatura comprendente almeno il 40% di stagno, almeno il 50% di indio o almeno il 50% di gallio ad una temperatura di almeno 380°C permette di effettuare una saldatura per diffusione in fase liquida che comporta la formazione di un composto intermetallico altofondente tra il rame e lo stagno (ad esempio Cu3Sn e/o Cu6Sn5), il gallio o l’indio. In particolare, la temperatura di fusione del composto intermetallico che si forma è maggiore della temperatura di fusione della lega di saldatura utilizzata. La formazione del composto intermetallico non coinvolge solo l’interfaccia tra il frame e la lega di saldatura o l’interfaccia tra la piastrina e la lega di saldatura, come nel caso del metodo di saldatura illustrato in US4170472, ma interessa l’intera massa della lega di saldatura applicata sul frame.
Vantaggiosamente è stato osservato che la formazione del composto intermetallico per diffusione del rame all’interno della massa della lega di saldatura consente di migliorare sensibilmente la resistenza meccanica del sistema costituito dalla piastrina e dal frame quando sottoposti a ciclatura termica rispetto ai sistemi ottenuti con i metodi di saldatura dell’arte nota. Ciò è dovuto al fatto che il composto intermetallico ottenuto, essendo altofondente, è meno sensibile al meccanismo di degrado per creep (scorrimento viscoso) rispetto alla lega di saldatura.
Vantaggiosamente, il metodo di saldatura sopra descritto richiede tempi molto inferiori rispetto a quelli dei metodi noti nell’arte per ottenere la diffusione del rame nella lega di saldatura, in particolare le fasi c) e d) richiedono nel loro complesso un tempo nell’intervallo di 0.1-10 secondi.
Ulteriori caratteristiche e vantaggi del presente metodo risulteranno dalla descrizione che segue di alcuni suoi esempi di realizzazione meramente illustrativi e non limitativi, riferiti ove occorre alle figure dei disegni annessi, in cui:
- le figure 1A-1H sono una rappresentazione schematica del presente metodo,
- le figure 2A-2B illustrano, rispettivamente, lo spettro EDX di una saldatura ottenuta mediante il presente metodo e lo spettro EDX di una saldatura ottenuta con una lega SnAgSb su un frame in nichel;
- le figure 3A-3D illustrano le immagini di microscopia acustica a scansione di una saldatura ottenuta mediante il presente metodo a confronto con una saldatura ottenuta con una lega SnAgSb su un frame in nichel (senza quindi diffusione del rame) ed una saldatura a base di piombo dell’arte nota dopo ripetuti cicli termici.
Il presente metodo verrà ora descritto con riferimento alle figure 1A-1H, che mostrano le fasi di una forma di realizzazione del metodo.
In fig. 1A, è raffigurato un supporto di connessione costituito da un leadframe 1 realizzato interamente in rame. Il leadframe 1 è disposto su un supporto 2. Il leadframe 1 è provvisto di una superficie 3. Nel caso in cui il leadframe 1 non sia interamente realizzato in rame, la superficie 3 viene rivestita con uno strato di rame.
Nella forma di realizzazione illustrata, il leadframe 1 viene riscaldato ad una temperatura di 380°C.
Come illustrato in figura 1B, sulla superficie 3 del leadframe 1 viene applicata una lega di saldatura 4, ad esempio consistente nel 65% stagno, 25% argento, 10% antimonio (65Sn-25Ag-10Sb) in modo analogo a quanto usualmente effettuato con la tecnica di saldatura con soft solder. A tale scopo, un filo di lega di saldatura 4 viene, attraverso una guida 5, applicato sulla superficie 3 del leadframe 1 riscaldato causando la fusione del filo di lega di saldatura 4 e la deposizione di una goccia 13 comprendente una quantità predeterminata di lega di saldatura 4 sulla superficie 3 (figura 1C) del telaio 1.
La goccia di lega di saldatura 13 così dispensata può, quindi, opzionalmente essere distesa uniformemente sulla superficie 3 mediante uno stampo 6 (figura 1D-1E).
Alternativamente, la lega di saldatura può essere applicata sulla superficie 3 del leadframe 1 in forma di solder preform che genera, a seguito di fusione a contatto con il leadframe 1 riscaldato, un volume ben definito di lega di saldatura 13 sulla superficie 3.
Successivamente, come illustrato nella figura 1F, una piastrina ″die″ 7, provvista di una superficie 8 rivestita con uno strato metallico 9, ad esempio di nichel, protetto da uno strato sacrificale di argento o oro, viene applicata sulla goccia di lega di saldatura 13 in modo che lo strato metallico 9 entri in contatto con tale goccia 13. La piastrina 7 è tipicamente di materiale semiconduttore, integra componenti elettronici e/o strutture microintegrate (non mostrati) realizzati con la tecnologia dei semiconduttori e forma, con la leadframe 1, un dispositivo integrato, ad esempio di potenza, indicato in figura 1H schematicamente con 100.
Quindi, l'insieme piastrina 7 - leadframe 2 così ottenuto viene lasciato raffreddare fino a completa saldatura (figura 1G).
Infine, figura 1H, elementi elettrici di interconnessione quali ad esempio fili elettrici di connessione 10 vengono saldati fra la piastrina 7 e gli adduttori (″pin″) 11 della leadframe 2, come mostrato schematicamente in figura 1H, fase di ″wire bonding″, e viene realizzato l'incapsulamento (″packaging″), ad esempio tramite stampaggio di un involucro (″package″) 12 di materiale isolante, ad esempio di resina (″molding compound″). Dopo l'esecuzione delle fasi finali di fabbricazione, quali indurimento (″curing″) dell'involucro 12, eventuale separazione degli adduttori 11, ecc., in modo noto al tecnico del ramo, si ottiene un dispositivo integrato 100.
Vantaggiosamente, il metodo illustrato nelle figure 1A-1H viene eseguito con macchinari comunemente noti ed utilizzati nei processi standard di “die attach”, utilizza materiali commercialmente disponibili e può essere utilizzato sia in linee produttive per dispositivi discreti che moduli di potenza.
Tale metodo è anche utilizzabile nel caso di montaggio a clip (″clip mounting″) in cui una struttura metallica (″clip″) viene saldata (brasata) sulla superficie superiore della piastrina 7. Inoltre, il procedimento descritto può essere utilizzato per saldare due strutture di connessione e/o supporto su due superfici opposte della piastrina.
Alternativamente, nel caso venga utilizzata la lega di saldatura come preform e laddove il leadframe 1 non venisse preventivamente riscaldato è possibile ottenere la fusione del preformato mediante riscaldamento in una fornace dopo che il preformato allo stato solido è stato disposto tra il connettore e la piastrina.
Vantaggiosamente il metodo consente l’ottenimento di una saldatura con uno spessore fino a 100 µm con proprietà termo-elettro-meccaniche migliorate rispetto a quelle ottenute con i metodi noti come evidenziato negli esempi che seguono.
ESEMPIO 1
Analisi di spettroscopia EDX (Energy Dispersive X-ray Analysis) di una saldatura ottenuta mediante il metodo dell’invenzione.
Una saldatura ottenuta mediante il metodo dell’invenzione con l’utilizzo della lega di saldatura 65Sn-25Ag-10Sb ed un frame in rame è stata analizzata mediante spettroscopia EDX e confrontata con gli spettri EDX ottenuti per saldature ottenute con la lega saldatura 65Sn-25Ag-10Sb su frame rivestiti con nichel. Gli spettri sono rispettivamente riportati nelle figure 2A-2B.
Come si può notare, nel caso di utilizzo di frame in rame nudo, si ha diffusione del rame in tutto lo spessore della lega di saldatura che crea un composto intermetallico con il materiale costituente la lega di saldatura. Nel caso di assemblaggio con frame in rame con finitura in Ni ciò non accade.
ESEMPIO 2
Affidabilità della saldatura ottenuta mediante il presente metodo
Una saldatura ottenuta mediante il presente metodo con l’utilizzo della lega di saldatura 65Sn-25Ag-10Sb ed un frame in rame è stata valutata in differenti test e confrontata con una saldatura effettuata con una lega 65Sn-25Ag-10Sb ed un frame con finitura in nichel. Un ulteriore confronto è stato effettuato con una lega a base piombo (95,5Pb-2Sn-2,5Ag) secondo l’arte nota.
I test effettuati su 25 campioni sono stati i seguenti: A.T.E. (Automatic Testing Equipment) = apparecchio automatico di test (TESEC), valutazione dei parametri elettrici statici del dispositivo (Resistenza di accensione ″on-resistance″, tensione di breakdown, correnti di leakage, tensioni di forward, etc) e delle relative deviazioni. Questo test è stato considerato fallito quando si hanno variazioni del più o meno 20% rispetto al valore misurato a tempo zero.
T.IMP.= deviazione dalla impedenza termica. Questo test è stato considerato fallito quando il campione ha mostrato un incremento dell’impedenza maggiore del 20% rispetto al valore iniziale.
S.A.M. (Scanning acoustic Microscopy) = Microscopia acustica a scansione (delaminazione fisica). Questo test è stato considerato fallito quando il campione ha mostrato una delaminazione fisica superiore al 20% rispetto al campione iniziale.
I risultati relativi al numero di fallimenti per ciascuna saldatura in ciascun test sono riportati nella tabella 1.
Tabella 1
La soluzione proposta mostra un beneficio in termini di performance affidabilistica con un fattore superiore a 5 rispetto all’arte nota.
Inoltre le figure 3A-3D riportano le immagini della SAM della saldatura ottenuta con la lega di saldatura 65Sn-25Ag-10Sb su un frame di rame, con la lega di saldatura 65Sn-25Ag-10Sb su un frame di nichel e con la lega di saldatura 95,5Pb-2Sn-2,5Ag su un frame di nichel.
Dal confronto sperimentale in ciclatura termica si evince come la soluzione con lega di saldatura 65Sn-25Ag-10Sb riesca a superare i 3000 cicli termici senza presentare alcuno scarto mentre a parità di condizioni ma con l’utilizzo di una frame con plating in Ni o con assemblaggio con pasta saldante in PbSnAg non si riescono a superare i 1000 cicli. Nelle figure 3B, 3C, 3D si evince il livello di degrado e delaminazione della solder in ciclatura in funzione del numero di cicli termici.
Risulta infine chiaro che al metodo e al dispositivo qui descritti ed illustrati possono essere apportate modifiche e varianti senza per questo uscire dall’ambito protettivo della presente invenzione, come definito nelle rivendicazioni allegate. Ad esempio, le diverse forme di realizzazione descritte possono essere combinate in modo da fornire ulteriori soluzioni.
Inoltre, sebbene il dispositivo mostrato venga assemblato mediante saldatura di elementi fili elettrici di connessione 10 fra la piastrina e i pin della leadframe e tramite incapsulamento tramite stampaggio di un involucro di materiale isolante, altre soluzioni possono essere previste, quali ad esempio fissaggio tramite clip (″clip attach″), o incapsulamento con involucri metallici o assemblaggi “potted” con l’utilizzo di gel siliconici.

Claims (8)

  1. RIVENDICAZIONI 1. Metodo per la saldatura di una piastrina (7) realizzata con la tecnica dei semiconduttori con un supporto di connessione (1) comprendente le fasi di: a) fornire un supporto di connessione (1) presentante almeno una superficie (3) realizzata almeno parzialmente in rame; b) fornire una piastrina (7) presentante almeno una superficie (8) rivestita con uno strato metallico (9); c) applicare a detta superficie (3) di detto supporto di connessione (1) una lega di saldatura (4) e scaldare detta lega (4) ad una temperatura di almeno 380°C per formare una goccia (13) di lega di saldatura in cui detta lega di saldatura è priva di piombo ed è scelta nel gruppo costituito da leghe comprendenti almeno il 40% in peso di stagno, leghe comprendenti almeno il 50% di indio e leghe comprendenti almeno il 50% di gallio; d) porre in contatto detto strato metallico (9) di detta piastrina (7) con detta goccia (13) di lega di saldatura per formare la saldatura con detto supporto di connessione (1).
  2. 2. Metodo secondo la rivendicazione 1, caratterizzato dal fatto che detto supporto di connessione (1) è realizzato interamente in rame.
  3. 3. Metodo secondo la rivendicazione 1, caratterizzato dal fatto che detta lega (4) è selezionata nel gruppo costituito da: - 61-69% stagno, 8-11% antimonio e 23-28% argento; - 50-99% stagno, 1-5% argento; - 98-98,9% stagno, 1-5% argento, 0,1-1% rame; - 50-99% indio, 1-50% argento; - 50-99% gallio, 1-50% nichel; - 50-99% gallio, 1-50% rame; - 70-99% gallio, 1-30% argento; le percentuali essendo espresse in peso sul peso totale della lega.
  4. 4. Metodo secondo la rivendicazione 3, caratterizzato dal fatto che detta lega (4) è 65% stagno, 25% argento, 10% antimonio, le percentuali essendo espresse in peso sul peso totale della lega.
  5. 5. Metodo secondo la rivendicazione 1, caratterizzato dal fatto che dette fasi c) e d) sono eseguite in un intervallo di tempo compreso tra 0.1 secondi e 10 secondi.
  6. 6. Metodo secondo la rivendicazione 1, caratterizzato dal fatto che detto strato metallico (9) è realizzato in un metallo scelto nel gruppo costituito da titanio, nichel, oro, argento, alluminio e rame.
  7. 7. Dispositivo (100), comprendente: un supporto di connessione (1) avente almeno una superficie realizzata in rame (3); una piastrina (7) realizzata con la tecnica dei semiconduttori ed avente almeno una superficie (8) rivestita con uno strato metallico (9); e una goccia di saldatura (13) disposta fra la superficie di rame (3) del supporto di connessione (1) e la superficie rivestita (8) della piastrina (7), in cui la goccia di saldatura (13) comprende una lega di saldatura (4) priva di piombo e scelta nel gruppo costituito da leghe comprendenti almeno il 40% in peso di stagno, leghe comprendenti almeno il 50% di indio e leghe comprendenti almeno il 50% di gallio e in cui la saldatura è realizzata secondo una qualsiasi delle rivendicazioni 1-6.
  8. 8. Dispositivo secondo la rivendicazione 7, comprendente un involucro (12) di materiale isolante circondante almeno parzialmente la piastrina (7) e il supporto di connessione (1).
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US16/811,964 US11482503B2 (en) 2019-03-11 2020-03-06 Method for the manufacture of integrated devices including a die fixed to a leadframe
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