Printed Circuit Design Tutorial (D. PCB finishing)
D. PCB finishing
The Plain copper on a printed circuit board oxidizes rapidly and has to be coated or plated with something to allow and maintain good solder ability. In the early years protective lacquers were used, roller Tinning quickly foll owed on along with Hot Air Solder Leveling (HAL; HASL), and the range of chemical finishes. In this section some regular finishing will be introduced.
1. HAL or Lead Free HAL (ROHS).
In this process the boards are fluxed then dipped into a large vertical solder tank. After which the excess solder is blown off with high temperature compressed air. This process produces a flat solder finish suitable for surface mount components.
Hot Air Solder Leveling produces one of the most stable end finishes with very good long term solder-ability. The shelf life of HAL usually can go up to be a few years. Lead free HAL actually works the same way as HAL, the only difference is that lead-free alloys are used in the process. Usually Lead free HAL will have a higher melt temp compared with regular leaded solders. So if lead free HAL is used as finishing which me ans that the material used to manufacture PCB will need to with stand have a much higher temp stress. For FR4 with Tg140 usually the max temp is 280 degree for 20 second, Tg170 will can works at 260 degree for 30 minutes. Of course the cost for TG170 material is higher, roughly 30% more than Tg140 material. Tg140 can be used in the lead free HAL process, but need works within its temp specification otherwise copper can lift up.
2. Immersion silver
The immersion silver finish provides a ROHS compliant finish for printed circuit boards (PCB) but is thinner than ENIG or traditiona l HASL finishes. Immersion silver is a newer PCB finish that offers better performance and superior finishes for final finishes on PCBs.
In this process silver is deposited directly on the copper surface by a chemical displacement reaction and is available in the industry all co-deposit and organic anti tarnish with the immersion silver. This reaction is fast and does not require the relatively high temperatures of ENIG. During assembly the immersion silver dissipates into the solder and allows the formation of a Cu/Sn intermetallic.
Immersion silver is an active surface and readily combines with sulfur from the environment. Silver sulfide tarnishes the surface and creates doubt about the integrity of the finish. The proper packaging of immersion silver finished boards are critical to control sulfurization. The key is to minimize contact of the surface with the environment and to ensure all materials used in the packaging and during storage are sulfur free.
Compared with HAL finishing, immersion silver offers a prefect flat surface and it is more popular in RF applications than HAL.
3. Gold Finishing (Electroless Nickel Immersion Gold)
Electroless nickel immersion gold (ENIG) is a type of surface plating used for printed circuit boards. It consists of electroless nickel plating covered with a thin layer of immersion gold, which protects the nickel from oxidatio n. The thickness of gold during this process usually is 3-5 micro inch, the thickness of Nick is around 150-250 micro inch.
Compared with HALENIG have a much higher cost. But ENIG has several advantages over more conventional (and cheaper) surface platings such as HASL (solder), including excellent surface planarity (particularly helpful for PCB's with large BGA packages), good oxidation resistance, and usability for untreated contact surfaces such as membrane switches and contact points. But consider solderbility HASL is over ENIG.
Organic Solderability Preservatives (OSPs), also known as anti-tarnish. OSP on bare copper printed circuit boards (PCBs) are becoming more prevalent in the electronics industry as the low-cost replacement to Hot Air Solder Leveling (HASL).
The biggest advantage of an OSP finished board is the price. OSP boards tend to be cheaper than those fabricated with other surfaces such as HASL, Immersion Tin, Immersion Silver or ENIG. Another advantage that OSP has, particularly as compared to HASL finished boards, is the flatness of the pads.
The primary drawbacks are related to solderabilit y, shelf life, re-use of misprinted boards, and multiple heating cycles. The solderability, shelf life issues are connected; if the OSP degrades through shelf life (typically less than 1 year), the Copper underneath oxidizes fairly easily and prevents the board from being generally easy to solder.
There-use of misprinted boards can also be an issue with OSP since alcohol and other solvents used to clean misp rinted boards will also remove the OSP, leaving the Copper surface very easy to oxidize. Misprinted boards th at are cleaned need to be sent back through the print/place/reflow processes quickly to avoid this issue.
Finally, OSP boards can be susceptible to solderability problems when subjected to multiple heating cycles; this occurs as a result of the OSP itself being degraded by the heating processes. There are new OSP materials that can overcome all of these issues.