As the industry has moved to lead-free assembly processing, demands on materials have increased, and suppliers are now utilizing phenolic based substrates due to higher Tg and Td values.
These new phenolic materials tend to pick up moisture over time. Moisture picked up around the periphery of the boards or around tooling holes that have not been plated or on exposed areas turns to steam during solder reflow. The vapor pressure of water can range from 300 psi for lower temperature solders (63% Tin 37% Lead), to over 700 psi at 260°C for lead-free solder assembly. This higher vapor pressure can cause delamination.
It is very important that the printed circuit boards are dry when they reach an assembly operation.
- You as the user must assure that your boards are not exposed to moisture during storage, board assembly, and rework. The boards should be placed in dry storage controlled at < 10% R.H.
- Special vacuum sealed packaging is available by SAE Circuits Colorado, Inc. along with moisture indicator strips to assure dryness. This is a value added service that SAE Circuits Colorado, Inc. provides for a small additional fee.
- Baking prior to solder reflow is required if:
- Boards have been stored in humid conditions (>60% R.H.) for any amount of time.
- Boards have remained in uncontrolled storage for over 10 days.
- Floor time life has been exceeded, and product is not stored with moisture indicators.
Please note: The baking does not necessarily guarantee assembly performance. The best practice is to avoid exposure to moisture.
If baking is required due to conditions indicated previously, SAE Circuits Colorado, Inc. recommends 250°F for 4 hours on all surface finishes. (With silver finish the silver may tarnish, but solderability should not be affected.)
Assembly Oven Profiling:
To avoid excessive over-temp conditions during processing, the assembly profile should be optimized depending on board thickness, area, and copper distribution.
As board thickness changes, the assembly profile may need to be adjusted by changing the line speed or the reflow oven zone temperatures in order to assure that all assemblies have the same temperature profile regardless of board thickness. Ramp rates should not exceed 1-2°C per second, keeping dwell times from above 125°C to reflow temps at a minimum.
Care must be taken to control and monitor the temperature gradient across the board areas during assembly. Unequal copper distribution and/or differential component mass can result in laminate “hot spots” or areas of excessive temperature that can result in delamination.
First Article Qualification:
Due to the variation in board designs, copper distribution, board thickness, board dimensions, and component type and placement, a First Article Qualification should be completed to verify the lead-free assembly process. Results of this qualification should be fed back to SAE Circuits Colorado, Inc. so that common design features and board construction options can be documented and implemented on similar and future board/assembly designs.
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