PCB Fabrication

The Importance Of Pcb Finishes

by:A-TECH      2020-10-26

Immersion gold or ENIG, the total name is Electroless nickel / Immersion gold, It’s a RoHS compliant finish that gives a really flat surface and a very dependable solder joint. HASL was the first end developed for SMOBC PCBs and it remains the primary end of choice for them. HASL is applied by immersing the completed SMOBC PCB in a vat of molten solder to coat the entire uncovered copper.

The PCB is drawn out of the solder bathtub through a pair of air knives that blow away the excess solder, leaving enough solder on every pad to offer corrosion protection. The drawback of this end is that the height of the solder pads is not uniform so fantastic pitch QFPs and BGAs ended up with solder shorts or open circuits. In addition, with HASL, every PCB is subjected to a extreme thermal shock when it's immersed within the molten solder.

With thick PCBs containing large numbers of small plated through-holes this can trigger some of these plated through-holes to fail. The results show a major advantage to using RAI gold compared with IG. IG showed main corrosion on the Ni layer, notably at the lower thickness of the electroless Pd layer. One method to decrease corrosion is to extend the thickness of the Pd layer. Test #1 involved various thicknesses of electroless Pd with standard IG.

However, course of engineers must additionally guard towards the galvanic effect. This situation arises when two dissimilar metals are involved with one another (corresponding to nickel and copper with selective ENIG processing, mainly copper-nickel gold interface). In addition, inner testing has proven that feature measurement, corresponding to large area gold pads instantly linked to copper traces, leads to extreme etching of the copper at the interface. By choosing immersion white tin as a pcb end, we are able to obtain the same impact of different finishes, however with lower influence to well being and surroundings. Basically the gold thickness of ENIG is 1-5U” and the nickel thickness is U”, the shelf life of immersion gold PCB can reach to 12 months.

Test #2 involved various thicknesses of electroless Pd with RAI gold. After every check, cross-sections of the ENEPIG layer at totally different Pd thicknesses have been evaluated for Ni corrosion utilizing SEM.

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