Q1: Our company have been soldering on Immersion Gold plated PCB’s for years without problems but now we experience many field failures with BGA/CSP type components. What is the problem?

A1: There can be many reasons but there is a high probability that the solder-joints are cracking because of high stress from mechanical shock.

BGA/CSP-type components are typically very stiff and significantly larger than SMD Resistors and Capacitors. Their solder-balls do not give stress relief as gull-wing type component pins does. Therefore BGA’s are more prone to fail than other SMD’s.

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High stress levels are created in the BGA/CSP balls when the PCB deflect because of mechanical impact.

The core of your problem is that you at all are soldering on Immersion Gold surface finish, combined with tough treatment of the device by the end user.

Immersion Gold (more correctly called Electroless Nickel Immersion Gold = ENIG) unfortunately has become a world standard PCB surface finish, but that doesn’t mean that ENIG is a good base for reliable solder-joints. Actually it’s very bad engineering and against well known material science to solder on ENIG. The problem is not the Gold as such because the immersion plating process by nature only deposit a very thin and porous layer. The problem is that the Nickel from the ENIG surface finish forms an Inter Metallic Compound (IMC) with Tin from the solder alloy (ball/paste).

This IMC is brittle like glass and therefore easily breaks when exposed to sudden mechanical stress as e.g. when you drop the device on the floor. Slow stress from Temperature Cycling seems not to give a reliability issue.

Beside use of a reliable PCB surface finish it is recommended to apply underfill under the BGA/CSP. This will glue the component safely to the PCB and at same time hinder moisture or water ingress to cause ElectroChemical Migration.


Conclusion:

- If you manufacture mobile devices that has a probability for being exposed to high mechanical stress then don’t ever solder on ENIG and assure that BGS/CSP's are underfilled !

- If you manufacture stationary equipment, soldering on ENIG could be argued to be sufficiently reliable – however it should never be recommended by material and process experts unless the BGA’s are underfilled and a lot of product specific analysis has verified if relevant use cases can be passed without reliability issues.

- The cheapest and most reliable surface finish for soldering is OSP.

Please contact E-Consult International to get professional advice.

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