Do you miss the days when your most reliable surface finish was HASL? Things were nice and easy, or at least selecting a solderable finish was nice and easy! Today’s process for selecting a solderable surface finish is not as simple. HASL, HAL, ENIG, ENEPIG, Immersion Silver, Immersion Tin, OSP ………with so many choices where do you start?
This could be one of the most important material decisions for the electronic assembly. The surface finish influences yield, rework rates, failure rates, the ability to test and of course, cost. Do not be lead astray by selecting the lowest cost finish. Selecting a surface finish should be done with a holistic approach that considers all important aspects of assembly.
A good place to start is to determine what is important to your company:
- Cost sensitivity
- Volume of product
- Lead or Lead Free process
- Shock/Drop a concern
- Cosmetics a concern
- User environment (is corrosion a concern?)
- Fine pitch assembly
- Wave solder required
- High yield ICT
There are advantages and disadvantages to each surface finish to be aware of and your best fit surface finish may be different depending on which criteria are most important to you.
The list below is far from complete but gives a start to the process of comparing various surface finishes:
Immersion Silver: One of the low cost options, this is a good finish for soldering and testing. Tarnish and creep corrosion are the top weaknesses. This is frequently used for fully enclosed hand held electronics and basic consumer electronics.
HT OSP: Another low cost option, this requires pasting of test pads/vias. Not all chemistries have the same performance and should be evaluated carefully. This finish is often used in hand held electronics, notebook and desk top computers, basic consumer electronics and power supplies.
LF HASL – This is a mid-cost option and there may some availability issues in the US. Phenolic laminate is recommended. Flatness is better than leaded HASL, but may not be ideal for fine pitch devices. This finish is often seen in business environments such as servers and telecom equipment.
Immersion Tin – Another mid-cost option. As an immersion finish, this performs well with fine pitch devices, but solderablility/hole fill may be a problem with double sided PCB’s. Shelf life is limited. This finish is often used in consumer electronics, simple medical and aerospace application and low volume peripheral components.
ENIG – One of the highest cost options, this is a good fit for fine pitch devices, handles multiple thermal cycles, has a long shelf life and solders easily. It is not gold wire-bondable, and is not optimal for higher speed signals. This is frequently used in medical and aerospace applications.
ENEPIG – This is the highest cost option. This finish is wirebondable, solderable and used mostly in the IC substrate market. This finish is often used for wirebonding applications, medical and aerospace products.
As I mentioned, this list is far from complete and meant to start discussion. There continues to be new advances in this field, so you will need to stay current on new developments. If you feel there is not a good option that meets your requirements, the chemistry suppliers are a great resource to make recommendations and provide options for “engineering” tricks to help improve on the weak areas of each finish.