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Advantages of Immersion Gold |
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Chemically speaking, gold is the most ideal element for external coating of PCB. Gold does not form oxides. Gold as an external coating will not be affected by temperature and storage conditions as other surface treatments, and reduce storage life. In addition, when welding, gold almost instantaneously melted into the solder and has excellent wettability. |
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The melting of gold into copper is very rapid. In order to prevent the fusion of gold from copper and eventually make the copper exposure oxidize to produce solderability problem, the gold and copper are separated by nickel plating. |
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So Electroless nickel immersion gold consists of a layer of electroless nickel plating covered with a thin layer of immersion gold. Normally, the thickness of nickel is 118μ” - 197μ” (3μm to 5μm) and gold 1-4μ” (0.025μm – 0.10μm). |
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Advantages
1) Flat Surface
The most important feature is that the surface of all pads is perfectly flat, corresponding to the underlying copper surface, with all pad and track edges covered by nickel/gold.? |
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2) Low Defect Rate
An important reason for choosing nickel/gold surface protection is a highly reduced failure rate during assembly and soldering compared with solder-coated and hot-air leveled boards. It is especially true of fine-line boards with a component pitch of 0.5 mm (20 mils) or less. |
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3) Solderability
Solderability is high but the soldering time is a little longer (about 5 sec.) compared with wave soldering (3 sec.). |
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4) Lead free / Pb free |
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5) Stressing of Boards
Because the boards have not been exposed to high temperatures, no stressing of the plated-through holes will occur. Under unfortunate conditions, this could otherwise lead to barrel and innerlayer cracks. Another advantage of the low-temperature processes is that no delamination of the laminate will take place.
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6) No copper melting and strong binding force of PTH |
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7) Dimensional Stability
Since the boards are not subjected to temperatures above 90° C (194° F) during manufacture, the dimensional stability is high. This is of great importance when screen-printing solder paste on fine-line SMT boards because a better fit between the stencil and the pattern is achieved than in the case of solder coated and hot-air leveled boards. |
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8) Contamination of Board Surface
Since there is no flux residue on the board surface as there is with solder-coated and hot-air leveled boards, surface contamination is considerably lower. |
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9) Fiducials
Fiducials, also called optical targets, achieve a better definition because of the thin nickel/gold layer. |
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10) Shelf Life
The shelf life is reported to be a minimum of one year when vacuum-packed. |
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11) Keyboard Contacts
It is expedient to execute on-board contacts for keyboards with the same nickel/gold coating as on the SMT pads so that a carbon printing can be avoided. |
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