Pre-Baking
Prior to Assembly Process
Pre-baking prior to assembly for flex and rigid-flex applications is extremely critical due to the hydroscopic nature of flexible PCB materials. We recommend the following pre-bake guidelines:| Pre-Baking Guidelines |
| HAND SOLDERING |
| Surfaces to be soldered need to be clean |
| Bake circuits at 250°F (121°C) for 1 hour |
| Remove enough units from the oven that can be soldered within 2 hours |
| Allow Cooling (if units not soldered in 2 hours, re-bake or place in desiccant) |
| Apply Flux (a rosin type flux like Kester 185 works well) |
| Use Acid Brushes to apply flux; area to be soldered should covered by the flux |
| A soldering iron (MetCal unit set at 700°F) with 700°F #2 solder tip ideal heat |
| Soldering tip held on the pad/component lead while applying the solder |
| A solder joint forms in 3-5 seconds |
| Minimize the dwell time |
| Longer dwell time causes solder to "run under" the covercoat or delaminate |
| Remove flux using an appropriate flux remover (vapor degreaser works ) |
| The solder joint should have nice uniform fillet with no voids or dewetting |
| Pre-bake for 45 minutes at 250°F |
| RIGID-FLEX WITH 2-LAYER FLEX |
| Pre-bake for 3 hours at 300°F |
| RIGID FLEX WITH MULTILAYER FLEX |
| Pre-bake for 4 hours at 325°F |
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To Bake or Not to Bake Yash published this article in SMT007 examining the impact of waiving Pre-Baking process prior to Assembly. Baking is critical to successful lead-free assemblies as it drives the moisture from the board. Yash addresses the "perfect storm" of reliability failures that include: moisture absorption, interlaminate adhesion strength, and water vapor pressure at lead-free assembly temperatures. Click here to read |