Elimination technology of PCB immersion silver layer

1. Hali ya sasa

Kila mtu anajua hilo kwa sababu printed mzunguko bodi haiwezi kufanywa upya baada ya kukusanyika, hasara ya gharama inayosababishwa na kufuta kutokana na microvoids ni ya juu zaidi. Ingawa wanane kati ya watengenezaji wa PWB waligundua kasoro kutokana na kurudi kwa wateja, kasoro kama hizo hukuzwa zaidi na mkusanyaji. Tatizo la solderability halijaripotiwa na mtengenezaji wa PWB hata kidogo. Wakusanyaji watatu pekee walidhani kimakosa tatizo la “kupungua kwa bati” kwenye ubao nene wa uwiano wa juu (HAR) wenye sinki/nyuso kubwa za joto (akimaanisha tatizo la kutengenezea mawimbi). Solder ya posta imejaa tu nusu ya kina cha shimo) kutokana na safu ya fedha ya kuzamishwa. Baada ya mtengenezaji wa awali wa vifaa (OEM) kufanya utafiti wa kina na uhakiki juu ya tatizo hili, tatizo hili linatokana kabisa na tatizo la solderability linalosababishwa na muundo wa bodi ya mzunguko, na haina uhusiano wowote na mchakato wa kuzamishwa kwa fedha au mwisho mwingine. njia za matibabu ya uso.

ipcb

2. Uchambuzi wa sababu ya mizizi

Through the analysis of the root cause of the defects, the defect rate can be minimized through a combination of process improvement and parameter optimization. The Javanni effect usually appears under the cracks between the solder mask and the copper surface. During the silver immersion process, because the cracks are very small, the silver ions supply here is limited by the silver immersion liquid, but the copper here can be corroded into copper ions, and then an immersion silver reaction occurs on the copper surface outside the cracks. . Because ion conversion is the source of the immersion silver reaction, the degree of attack on the copper surface under the crack is directly related to the thickness of the immersion silver. 2Ag++1Cu=2Ag+1Cu++ (+ is a metal ion that loses an electron) cracks can be formed for any of the following reasons: side corrosion/excessive development or poor bonding of the solder mask to the copper surface; uneven copper electroplating layer (hole Thin copper area); There are obvious deep scratches on the base copper under the solder mask.

Corrosion is caused by the reaction of sulfur or oxygen in the air with the metal surface. The reaction of silver and sulfur will form a yellow silver sulfide (Ag2S) film on the surface. If the sulfur content is high, the silver sulfide film will eventually turn black. There are several ways for silver to be contaminated by sulfur, air (as mentioned above) or other pollution sources, such as PWB packaging paper. The reaction of silver and oxygen is another process, usually oxygen and copper under the silver layer react to produce dark brown cuprous oxide. This kind of defect is usually because the immersion silver is very fast, forming a low-density immersion silver layer, which makes the copper in the lower part of the silver layer easy to contact with the air, so the copper will react with the oxygen in the air. The loose crystal structure has larger gaps between the grains, so a thicker immersion silver layer is needed to achieve oxidation resistance. This means that a thicker silver layer must be deposited during production, which increases production costs and also increases the probability of solderability problems, such as microvoids and poor soldering.

Mfiduo wa shaba kawaida huhusiana na mchakato wa kemikali kabla ya kuzamishwa kwa fedha. Hitilafu hii inaonekana baada ya mchakato wa kuzamishwa kwa fedha, hasa kwa sababu filamu iliyobaki ambayo haijaondolewa kabisa na mchakato uliopita inazuia uwekaji wa safu ya fedha. Ya kawaida zaidi ni filamu iliyobaki inayoletwa na mchakato wa mask ya solder, ambayo husababishwa na maendeleo yasiyo safi katika msanidi programu, ambayo ni kinachojulikana kama “filamu iliyobaki”. Filamu hii iliyobaki inazuia athari ya fedha ya kuzamishwa. Mchakato wa matibabu ya mitambo pia ni moja ya sababu za udhihirisho wa shaba. Muundo wa uso wa bodi ya mzunguko utaathiri usawa wa mawasiliano kati ya bodi na suluhisho. Mzunguko wa kutosha au mwingi wa suluhisho pia utaunda safu ya kuzamishwa kwa fedha isiyo na usawa.

Uchafuzi wa ioni Dutu za ioni zilizopo kwenye uso wa bodi ya mzunguko zitaingilia utendaji wa umeme wa bodi ya mzunguko. Ioni hizi hasa hutoka kwenye kioevu cha kuzamishwa kwa fedha yenyewe (safu ya kuzamisha ya fedha inabaki au chini ya mask ya solder). Suluhu tofauti za fedha za kuzamishwa zina maudhui tofauti ya ioni. Kadiri kiwango cha ioni kilivyo juu, ndivyo thamani ya uchafuzi wa ioni inavyoongezeka chini ya hali sawa za kuosha. Porosity ya safu ya fedha ya kuzamishwa pia ni moja ya mambo muhimu yanayoathiri uchafuzi wa ioni. Safu ya fedha yenye porosity ya juu ni uwezekano wa kuhifadhi ions katika suluhisho, ambayo inafanya kuwa vigumu zaidi kuosha na maji, ambayo hatimaye itasababisha ongezeko linalofanana la thamani ya uchafuzi wa ion. Athari ya baada ya kuosha pia itaathiri moja kwa moja uchafuzi wa ioni. Uoshaji duni au maji yasiyo na sifa yatasababisha uchafuzi wa ioni kuzidi kiwango.

Microvoids kawaida huwa chini ya 1mil kipenyo. Voids iko kwenye kiwanja cha interface ya chuma kati ya solder na uso wa soldering huitwa microvoids, kwa sababu kwa kweli ni “cavities ya ndege” kwenye uso wa soldering, hivyo hupunguzwa sana. Nguvu ya kulehemu. Uso wa OSP, ENIG na fedha ya kuzamishwa itakuwa na microvoids. Sababu ya mizizi ya malezi yao haijulikani, lakini mambo kadhaa ya ushawishi yamethibitishwa. Ingawa microvoids zote kwenye safu ya fedha ya kuzamishwa hutokea kwenye uso wa fedha nene (unene unaozidi 15μm), sio tabaka zote za fedha nene zitakuwa na microvoids. Wakati muundo wa uso wa shaba chini ya safu ya fedha ya kuzamishwa ni mbaya sana, microvoids ni uwezekano mkubwa wa kutokea. Kutokea kwa microvoids pia inaonekana kuwa inahusiana na aina na muundo wa vitu vya kikaboni vilivyowekwa pamoja kwenye safu ya fedha. Kwa kukabiliana na jambo lililo hapo juu, watengenezaji wa vifaa vya asili (OEM), watoa huduma wa utengenezaji wa vifaa (EMS), watengenezaji wa PWB na wasambazaji wa kemikali walifanya tafiti kadhaa za kulehemu chini ya hali zilizoigizwa, lakini hakuna hata mmoja wao anayeweza kuondoa kabisa microvoids.