Medical Electronics PCB Assembly Board Baking Conditions and Specifications

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1. Baking conditions and time settings for medical electronic PCBs
2. Stacking method during baking of medical electronic PCBs
3. Precautions during baking of medical electronic PCB boards
The primary purpose of PCB baking is to remove moisture and humidity, eliminating water vapour contained within the PCB or absorbed from the external environment.
This is because certain materials used in PCB construction are inherently prone to forming water molecules.

Conditions and Duration for Baking Medical Electronics PCBs

1. PCBs manufactured within the last two months and well-sealed, which have been opened and stored in a temperature- and humidity-controlled environment (≤30°C/60% RH, per IPC-1601) for over five days, must be baked at 120±5°C for one hour prior to assembly.
2. PCBs stored for 2 to 6 months beyond the manufacturing date must be baked at 120±5°C for 2 hours prior to use;
3. PCBs stored for 6 to 12 months beyond the manufacturing date must be baked at 120±5°C for 4 hours prior to use;
4. PCBs stored for over 12 months beyond the manufacturing date are generally not recommended for use. The bonding strength of multilayer boards degrades over time, potentially causing unstable product functionality and quality issues, increasing the likelihood of market returns.
Additionally, production risks include board delamination and solder wetting defects. Should use be unavoidable, it is recommended to first bake at 120±5°C for 6 hours. Prior to mass production, conduct a trial solder paste printing on several boards to confirm no solderability issues before proceeding with production.

Stacking Method for Medical Electronics PCBs During Baking

1. For large-sized PCBs during baking, adopt a flat stacking arrangement. It is recommended that each stack contain no more than 30 pieces. Within 10 minutes of baking completion, the oven must be opened to remove the PCBs and lay them flat for cooling. Post-baking, apply anti-warping jigs to prevent bending. Vertical stacking is not advised for large PCBs as it increases susceptibility to warping.
2. For small and medium-sized PCBs, flat stacking is permissible with a maximum recommended stack size of 40 pieces. Vertical stacking is also acceptable with no quantity restriction. Within 10 minutes of baking completion, remove PCBs from the oven and lay them flat to cool. Post-baking, apply pressure to prevent warping.

Precautions for Baking Medical Electronic PCB Boards

1. Baking temperature must not exceed the PCB's glass transition temperature (Tg), typically not exceeding 125°C. Early lead-containing PCBs had lower Tg values, whereas most modern lead-free PCBs have Tg above 150°C;
2. Bake-dried PCBs should be utilised promptly. Any unused stock must be re-vacuum-sealed without delay. PCBs exposed in the workshop for extended periods require re-baking;
3. Ensure ovens are fitted with extraction drying equipment. Failure to do so allows moisture vapour to accumulate within the oven, increasing relative humidity and hindering PCB dehumidification;
4. From a quality perspective, the fresher the PCB, the better the solder rework quality. Expired PCBs retain inherent quality risks even after baking;
5. All baked PCBs must be utilised within five days. Unprocessed PCBs must be rebaked at 120±5°C for one hour prior to assembly.

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