Intro to Surface Finishes For Flexible Circuits(2)


Immersion silver is an additional surface candidate. Silver is one of the most conductive aspects. Also its oxides are extremely conductive, however sadly are not as conveniently solderable. There are likewise long-lasting problems over silver migration. Strides appear to have actually been made in the last few years to address both concerns. Immersion finishes as thin as 0.3 mm seem efficient in keeping solderability as well as have the included benefit of being wire-bondable utilizing light weight aluminum cord ultrasonic bonding methods.


Electroless nickel immersion gold (ENIG) finishes have been the topic of many study papers in recent times. They have been displayed as a cure all of types, providing in a solitary finish, planarity, solderability, cable bondability and then a corrosion-resistant get in touch with coating, so all the initiative making them function reliably comes as no surprise. What has actually been problematic is the occasional occurrence of the black pad sensation in which solder leads lift away from what was the nickel-gold interface. Nonetheless, some study has recommended that a higher percent of phosphorous in the nickel might relieve the problem.

Immersion gold using the ENIG process is usually 0.1 µm to 0.2 µm thick and then not appropriate to gold round bonding or a number of insertion connections, making electrolytic gold another alternative. For gold sphere bonding, a density of ~ 1.0 µm has actually verified appropriate. Soft gold operating a firmness of approximately 90 Knoop is chosen. For calls, hard gold plating is chosen. Electrolytic gold needs that electrical links be present and shorting bars are typically utilized to this end.


Another coating that has actually acquired some focus is direct immersion gold (DIG). DIG has the advantage that there is no nickel plating called for, making it an easier process. Immersion layer of gold straight over copper is a straightforward process because both metals are much apart in the electromotive series. Gold will conveniently change copper, yet the density is restricted by diffusion to less than a micrometer. The one big drawback is that gold is presently floating around $1500 per troy ounce, making it a costly and much less appealing solution.

Organic solderability safety layers (OSP) have a lengthy record, going back nearly to the start of the PCB sector. The existing course of finishes is far superior to the earlier products. More recent coatings fast as well as simple to use as well as have been developed to make it through numerous travel through assembly while securing as well as preserving the solderability of the copper they layer. OSPs have been successful for standard solders, but some problem has actually been expressed regarding their capacity to survive as numerous passes in flex PCB lead-free handling.

Finish selections are plentiful to meet the various needs of the myriad of applications that flexible circuits now serve. Along with the selections detailed, there have been some fascinating finishes based on nanotechnology that have been seeing some greater usage. Nonetheless, without any true cure all in the mix, the choice should be made based on the assembly as well as use requirements of the design.

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