The Effects of Lead-Free on PCB Fabrication
Currently there is a strong movement for lead-free solder on PCB fabrication. This soldering method requires submitting a high temperature to the substances and their finishes. This production of a new generation of PCB materials requires 260 degrees C. There is a high reliability type FR-4 called phenolic cure materials that can withstand higher temperatures. Nine other lead compatible materials are being produced by several companies. Some of these have resins that are more heat resistant but the final product is more costly. An assessment is needed to compare electrical performance against the production cost. Revisions in design should be considered in this regard.
There are six PCB surface finishes available for treatment:
• immersion tin
• immersion silver
• organic solderability preservative (OSP)
• electrolytic nickel/gold
• electroless nickel/immersion gold (ENIG)
Each finish has its pluses and minuses. A silver-based finish offers an effortless surface to solder but its shelf life is shorter. These existing surface finishes too may require outsourcing to make the necessary adjustments resulting in additional monetary expenditures, time extensions and quality control issues. The availability of manufacturers is limited too. Those who have converted to lead-free in the global arena are only 17% since the conversion cost is extremely expensive.
The RoHS compliance involves a testing process of several new materials. New lead-free soldering procedures must be established to ascertain their soldering guidelines.
Copper, tin and silver alloys are the prime choices that can withstand multiple 260 degrees C. without changing reliability.
Fabricators of new materials entering the market must ascertain the CAF growth. Also known as conductive anodic filament growth, this must be ascertained to conclude if the material can serve as the sought after replacement for the customary FR-4 practices.
Certain factors must be taken into account when selecting materials. Higher temperatures risk delamination because through hole reliability has been jeopardized. Reliability is worrisome due to higher aspect ratios. Increased thermal mass affects the safety of reflow equipment. The temperature of glass transition for the resin procedure is when the material becomes soft. When temperatures are higher it is increasingly more important to pay attention to the CTE, otherwise known as the coefficient of thermal expansion and decomposition temperature. These materials can only tolerate high temperatures for a limited time. Chemical bonds will eventually sever.
It should be ascertained if the finish can withstand multiple reflow cycles. Solder joint reliability may be compromised since some finishes are prone to deteriorating more quickly at lead-free assembly temperatures.
If the glass transition temperature is rated at more than 170 degrees Celsius there is the potential for delamination to occur. Support points too may need to be added on the board layout for board retention.
The board may need to have some clear markings to establish the lead-free designation mark.
The guidelines for the component checklist should include the materials to be implemented in the component construction, the temperature range of 220 degrees C for 90 seconds and no higher than 260 degrees C. The finish should be selected from this list:
In addition the following should be addressed:
• JEDEC moisture level should be monitored
• Component performance should be confirmed
• Finish solderability should be confirmed
• An ID protocol should be established for marking non lead components
Keeping the conductive anodic filament growth in mind, unconverted components will be disposed of and document changes will be checked for accurate corrections. Finally a timeline for implementation of required changes will be needed.
All markets will eventually have to transfer to a non-lead model. This presents a predicament for the military since the military has not had to comply with the RoHS non-lead soldering conditions. As manufacturing of the alternative components takes place these will be the only soldered products available. There will be a kind of domino effect when the RoHs becomes widespread.
Designers must realize at all times that some materials will not tolerate the higher temperatures. In order to establish that the transition be as seamless as possible, all parties concerned must work together.