Txheej txheej txheej zaum kawg rau PCB kev tsim khoom tau dhau qhov kev hloov pauv tseem ceeb hauv xyoo tas los no. These changes are the result of the constant need to overcome the limitations of HASL(Hot air cohesion) and the growing number of HASL alternatives.

Cov txheej txheej kawg yog siv los tiv thaiv cov nplaim hluav taws xob tooj liab. Tooj liab (Cu) yog qhov chaw zoo rau cov khoom siv vuam, tab sis yooj yim oxidized; Cov tooj liab oxide cuam tshuam rau ntub cov khoom ntawm cov laug. Txawm hais tias kub (Au) tam sim no siv los npog tooj liab, vim tias kub tsis oxidize; Kub thiab tooj liab yuav nthuav tawm sai thiab nkag mus rau ib leeg. Ib qho tooj liab uas tau nthuav tawm yuav tsim sai sai uas tsis tuaj yeem txuas tau tooj liab oxide. Ib txoj hauv kev yog siv cov xaum npib tsib xee (Ni) “txheej txheej” uas tiv thaiv kub thiab tooj liab los ntawm kev hloov pauv thiab muab cov khoom ruaj khov, ua kom zoo rau cov khoom sib dhos.

PCB cov txheej txheem rau txheej txheej uas tsis yog electrolytic nickel

The non-electrolytic nickel coating should perform several functions:

Qhov saum npoo ntawm qhov tso nyiaj kub

Lub hom phiaj kawg ntawm kev sib tw yog tsim kom muaj kev sib txuas nrog lub cev muaj zog thiab cov yam ntxwv hluav taws xob zoo ntawm PCB thiab cov khoom siv. Yog tias muaj oxide lossis paug ntawm PCB saum npoo, qhov sib txuas ua ke yuav tsis tshwm sim nrog niaj hnub no tsis muaj zog.

Cov tso nyiaj kub ib txwm nyob saum nickel thiab tsis oxidize thaum khaws cia ntev. Txawm li cas los xij, kub tsis nyob ntawm cov xaum npib tsib xee oxidized, yog li cov npib tsib xee yuav tsum nyob dawb huv ntawm cov xaum npib tsib xee thiab ua kom kub ploj mus. Thus, the first requirement of nickel is to remain oxygen-free long enough to allow gold to precipitate. Components developed chemical leaching baths to allow 6~10% phosphorus content in nickel precipitation. This phosphorus content in the non-electrolytic nickel coating is considered as a careful balance of bath control, oxide, and electrical and physical properties.

hardness

Cov tsis-electrolytic npib tsib xee txheej txheej tau siv rau hauv ntau daim ntawv thov uas xav tau lub cev muaj zog, xws li tsheb thauj mus los. Cov kev cai PCB tsis nruj me ntsis ntau dua li cov ntawv thov no, tab sis qee qhov nyuaj yog qhov tseem ceeb rau xaim-sib txuas, kov cov neeg sib tham, kov-txuas txuas txuas, thiab ua kom ruaj khov.

Kev sib txuas ua txhuas xav tau cov xim nickel hardness. Kev poob ntawm kev sib txhuam tuaj yeem tshwm sim yog tias cov hlau lead deforms nag lossis daus, uas pab cov hlau lead “yaj” rau hauv txheej txheej. SEM cov duab pom tsis muaj kev nkag mus rau saum npoo ntawm npib tsib xee/kub lossis nickel/palladium (Pd)/kub.

Cov yam ntxwv hluav taws xob

Tooj liab yog cov hlau xaiv rau tsim hluav taws xob vim tias nws yooj yim los ua. Tooj liab ua hluav taws xob zoo dua yuav luag txhua lub hlau (rooj 1) 1,2. Gold also has good electrical conductivity, making it a perfect choice for the outermost metal because electrons tend to flow on the surface of a conductive route (the “surface” benefit).

Table 1. Resistivity of PCB metal

Tooj 1.7 (suav nrog Ω cm

Gold (including 2.4 Ω cm

Nickel (including 7.4 Ω cm

Tsis-electrolytic npib tsib xee txheej 55 ~ 90 µ ω cm

Although the electrical characteristics of most production plates are not affected by the nickel layer, nickel can affect the electrical characteristics of high frequency signals. Microwave PCB teeb liab poob tuaj yeem tshaj qhov tsim tshwj xeeb. This phenomenon is proportional to the thickness of the nickel – the circuit needs to pass through the nickel to reach the solder spot. Hauv ntau daim ntawv thov, cov cim hluav taws xob tuaj yeem rov qab mus rau qhov tsim tshwj xeeb los ntawm kev qhia cov xaum npib tsib xee tsawg dua 2.5µm.

Hu tau tiv

Kev tiv tauj tiv thaiv sib txawv los ntawm weldability vim tias cov xaum npib tsib xee/kub saum npoo tseem tsis zoo nkauj thoob plaws lub neej ntawm cov khoom kawg. Npib tsib xee/kub yuav tsum nyob twj ywm rau kev sib cuag sab nraud tom qab raug ib puag ncig ntev. Antler’s 1970 book expressed nickel/gold surface contact requirements in quantitative terms. Various end-use environments have been studied: 3 “65°C, a normal maximum temperature for electronic systems operating at room temperature, such as computers; 125 ° C, qhov kub thiab txias uas txhua qhov txuas sib txuas yuav tsum ua haujlwm, feem ntau tau teev tseg rau kev siv tub rog; 200 ° C, qhov kub ntawd tau dhau los ua qhov tseem ceeb rau cov khoom siv ya. “

Rau qhov kub qis, tsis muaj teeb meem npib tsib xee. As the temperature increases, the amount of nickel required to prevent nickel/gold transfer increases (Table II).

Rooj 2. Tiv tauj tsis kam ntawm npib tsib xee/kub (1000 teev)

Npib tsib xee txheej txheej txaus siab hu rau ntawm 65 ° C txaus siab hu rau ntawm 125 ° C txaus siab hu rau ntawm 200 ° C

0.0 µm 100% 40% 0%

0.5 µm 100% 90% 5%

2.0 µm 100% 100% 10%

4.0 µm 100% 100% 60%

Cov npib tsib xee siv hauv Antler txoj kev tshawb fawb tau ua hluav taws xob. Kev txhim kho tau cia siab los ntawm cov xaim hluav taws xob uas tsis yog hluav taws xob, raws li tau lees paub los ntawm Baudrand 4. Txawm li cas los xij, cov txiaj ntsig no yog rau 0.5 µm kub, qhov twg lub dav hlau feem ntau ua rau 0.2 µm. Lub dav hlau tuaj yeem txiav txim siab kom txaus rau cov khoom sib txuas ua haujlwm ntawm 125 ° C, tab sis cov ntsiab lus kub ntau dua yuav xav tau kev kuaj tshwj xeeb.

Antler qhia tias “Cov tuab tuab npib tsib xee, qhov thaiv zoo dua, hauv txhua kis,” tab sis qhov tseeb ntawm PCB kev tsim khoom txhawb cov kws tsim khoom kom tso nyiaj ntau npaum li npib tsib xee raws li xav tau. Tam sim no npib tsib xee/kub tam sim no tau siv hauv xov tooj ntawm tes thiab pagers uas siv cov kov-ncoo cov ntsiab lus. The specification for this type of element is at least 2 µm nickel.

Qhov txuas

Cov tsis-electrolytic npib tsib xee/kub immersion tau siv hauv kev tsim cov phiajcim hluav taws xob nrog lub caij nplooj ntoo hlav haum, nias-haum, qis-siab zawv zawg thiab lwm yam tsis txuas txuas.

Plug-in txuas xav tau lub cev ntev dua. Hauv cov xwm txheej no, cov xim tsis xaim hluav taws xob nickel muaj zog txaus rau PCB daim ntawv thov, tab sis kub raus dej tsis yog. Very thin pure gold (60 to 90 Knoop) will rub away from the nickel during repeated friction. Thaum cov kub raug tshem tawm, cov npib tsib xee raug oxidizes sai, ua rau muaj kev tiv thaiv kev tiv thaiv ntau ntxiv.

Non-electrolytic nickel coating/gold immersion may not be the best choice for plug-in connectors that endure multiple inserts throughout the life of the product. Npib tsib xee/palladium/kub nto tau pom zoo rau cov khoom sib txuas sib txuas.

The barrier layer

Non-electrolytic nickel has the function of three barrier layers on the plate: 1) to prevent the diffusion of copper to gold; 2) To prevent the diffusion of gold to nickel; 3) Qhov chaw ntawm npib tsib xee tsim los ntawm Ni3Sn4 intermetallic sib txuas.

Diffusion tooj liab rau npib tsib xee

Kev hloov tooj liab los ntawm npib tsib xee yuav ua rau decomposition ntawm tooj liab mus rau saum npoo kub. The copper will oxidize quickly, resulting in poor weldability during assembly, which occurs in the case of nickel leakage. Nickel is needed to prevent migration and diffusion of empty plates during storage and during assembly when other areas of the plate have been welded. Yog li ntawd, qhov xav tau kub ntawm txheej txheej thaiv yog tsawg dua ib feeb hauv qab 250 ° C.

Tig thiab Owen6 tau kawm txog qhov cuam tshuam ntawm txheej txheej sib txawv ntawm tooj liab thiab kub. Lawv pom tias “… Kev sib piv ntawm tooj liab permeability qhov tseem ceeb ntawm 400 ° C thiab 550 ° C qhia tias hexavalent chromium thiab npib tsib xee nrog 8-10% cov ntsiab lus phosphorus yog cov txheej txheem thaiv zoo tshaj plaws “. (rooj 3).

Rooj 3. Kev nkag ntawm tooj liab los ntawm npib tsib xee rau kub

Nickel thickness 400°C 24 hours 400°C 53 hours 550°C 12 hours

0.25 µm 1 µm 12 µm 18 µm

0.50 µm 1 µm 6 µm 15 µm

1.00 µm 1 µm 1 µ M 8 µm

2.00 µm Non-diffusion non-diffusion non-diffusion

According to the Arrhenius equation, diffusion at lower temperatures is exponentially slower. Qhov txaus siab, hauv qhov kev sim no, cov xaim hluav taws xob tsis muaj electrolytic yog 2 txog 10 zaug ntau dua li cov xaim hluav taws xob nickel. Tig thiab Owen taw qhia tias “… A (8%) 2µm(80µinch) barrier of this alloy reduces copper diffusion to a negligible level.”

Los ntawm qhov ntsuas kub hnyav no, tuab npib tsib xee ntawm tsawg kawg 2µm yog qhov ua tau zoo.

Diffusion ntawm npib tsib xee rau kub

Qhov thib ob qhov xav tau ntawm cov xaim hluav taws xob uas tsis yog hluav taws xob yog tias npib tsib xee tsis txav los ntawm “cov nplej” lossis “qhov zoo” uas tsis muaj xim kub. If nickel comes into contact with air, it will oxidize. Nickel oxide is not soldable and difficult to remove with flux.

Muaj ob peb tsab xov xwm ntawm npib tsib xee thiab kub siv los ua cov khoom siv ntim khoom ntim khoom ntim rau hauv ceramic. These materials withstand the extreme temperatures of assembly for a long time. Ib qho kev sim rau cov chaw no yog 500 ° C rau 15 feeb.

Txhawm rau ntsuas qhov muaj peev xwm ntawm cov tiaj tus uas tsis muaj electrolytic npib tsib xee/kub-impregnated los tiv thaiv npib tsib xee oxidation, weldability ntawm kub-hnub nyoog qhov chaw tau kawm. Different heat/humidity and time conditions were tested. Cov kev tshawb fawb no tau qhia tias npib tsib xee muaj kev tiv thaiv zoo los ntawm kev xeb kub, tso cai rau weldability zoo tom qab laus laus.

Kev sib xyaw ntawm npib tsib xee rau kub tuaj yeem yog qhov txwv rau kev sib dhos hauv qee kis, xws li kub thermalsonic xaim-sib txuas. Hauv daim ntawv thov no, npib tsib xee/kub saum npoo yog tsawg dua li npib tsib xee/palladium/kub nto. Iacovangelo investigated the diffusion properties of palladium as a barrier layer between nickel and gold and found that 0.5µm palladium prevents migration even at extreme temperatures. This study also demonstrated that there was no diffusion of copper through 2.5µm of nickel/palladium determined by Auger spectroscopy during 15 minutes at 500°C.

Nickel tin intergeneric compound

During surface mount or wave soldering operation, atoms from the PCB surface will be mixed with solder atoms, depending on the diffusion properties of the metal and the ability to form “intermetallic compounds” (Table 4).

Rooj 4. Diffusivity ntawm PCB cov ntaub ntawv hauv vuam

Hlau kub ° C diffusivity (µinches/ SEC.)

Kub 450 486 117.9 167.5

Tooj 450 525 4.1 7.0

Palladium 450 525 1.4 6.2

Nickel 700 1.7

Hauv npib tsib xee/kub thiab tin/cov txheej txheem, cov kub tam sim ntawd yaj mus rau hauv xoob xoob. The solder forms a strong attachment to the underlying nickel by forming Ni3Sn4 intermetallic compounds. Enough nickel should be deposited to ensure that the solder will not reach underneath the copper.Bader kev ntsuas pom tias tsis ntau tshaj 0.5µm ntawm npib tsib xee tau xav kom tswj hwm qhov teeb meem, txawm tias dhau ntau dua rau rau lub hlis mus los. In fact, the maximum intermetallic layer thickness observed is less than 0.5µm(20µinch).

dej txeem

Tsis-electrolytic npib tsib xee/kub tsuas yog nyuam qhuav dhau los ua qhov kawg PCB txheej txheej, yog li cov txheej txheem kev tsim khoom yuav tsis haum rau saum npoo no. Cov txheej txheem nitric acid muaj nyob rau kuaj qhov porosity ntawm electrolytic npib tsib xee/kub siv ua qhov txuas txuas (IPC-TM-650 2.3.24.2) 9. Cov tsis xaim hluav taws xob nickel/impregnation yuav tsis dhau qhov kev xeem no. Cov txheej txheem porosity nyob sab Europe tau tsim los siv cov tshuaj potassium ferricyanide los txiav txim siab qhov txheeb ze porosity ntawm cov tiaj, uas tau muab raws li qhov pores ib square millimeter (kab /mm2). Ib lub tiaj tiaj zoo yuav tsum muaj tsawg dua 10 qhov ntawm ib square millimeter ntawm 100 x kev nthuav dav.

xaus

PCB kev tsim khoom lag luam txaus siab txo qis cov nyiaj npib tsib xee tso rau ntawm pawg thawj coj saib vim li cas tus nqi, lub sijhawm hloov pauv, thiab cov khoom sib xws. Qhov tsawg tshaj plaws ntawm cov xaum npib tsib xee yuav tsum pab tiv thaiv tooj liab kis mus rau qhov kub, tuav lub zog txuas kom zoo, thiab ua kom muaj kev sib tiv tauj tsawg. Qhov siab tshaj plaws ntawm npib tsib xee yuav tsum tso cai hloov pauv hauv kev tsim phaj, vim tias tsis muaj hom kev ua tsis tiav yog cuam tshuam nrog cov npib tsib xee tuab.

Rau feem ntau ntawm cov phiaj xwm niaj hnub no, cov xim nickel tsis-electrolytic txheej ntawm 2.0µm (80µinches) yog qhov tsawg kawg ntawm npib tsib xee tuab uas xav tau. Hauv kev ua haujlwm, ntau yam ntawm cov tuab tuab npib tsib xee yuav siv rau ntau lawm ntawm PCB (Daim duab 2). Kev hloov pauv ntawm npib tsib xee tuab yuav tshwm sim los ntawm kev hloov pauv hauv cov khoom siv tshuaj lom neeg da dej thiab kev hloov pauv nyob hauv lub sijhawm nyob ntawm lub tshuab nqa tsis siv neeg. Yuav kom ntseeg tau qhov tsawg kawg ntawm 2.0µm, kev qhia tshwj xeeb los ntawm cov neeg siv kawg yuav tsum xav tau 3.5µm, yam tsawg kawg ntawm 2.0µm, thiab qhov siab tshaj ntawm 8.0µm.

Qhov tshwj xeeb ntau ntawm cov xim npib tsib xee tau ua pov thawj tsim nyog rau kev tsim ntau lab ntawm cov laug cam. Qhov ntau ua tau raws li weldability, lub neej txee thiab kev xav tau ntawm cov khoom siv hluav taws xob niaj hnub no. Vim tias kev sib dhos xav tau sib txawv ntawm ib yam khoom mus rau lwm qhov, txheej txheej txheej yuav tsum tau ua kom zoo dua rau txhua qhov kev thov tshwj xeeb.