Foaming occurred in densely buried holes under the HDI laminate. This phenomenon occurred in 2006, and there was no lead-lead introduction period. At that time, the board manufacturer did not have experience in dealing with lead-free, thinking that high Tg sheets could be used. After analyzing and researching the pore-filling resin, the amount of rubber in the prepreg, and the process, it was found that the main problem was caused by the moisture absorption of the sheet.
High T, the plate is easy to absorb moisture, because the effect of moisture absorption on the welding after the increase of lead-free temperature is insufficient. No corresponding moisture-proof measures have been taken, resulting in batch delamination of lead-free soldering.
This case reminds us that the increase in lead-free soldering temperature makes the PCB a moisture sensitive material. In the manufacturing process of PCB manufacturing, SMT welding, etc., the moisture absorption problem must be strictly controlled.
The countermeasures for the above reasons are:
1) PCB manufacturing process
(1) Use a high T, sheet (150 ~ 170 ° C), low CTE hole-filling resin and high gel content (= 65%) suitable for lead-free process.
(2) Strengthen the process moisture control, increase the baking process before lamination and before packaging.
(3) Packed in airtight aluminum foil.
(1) Strictly control over-time on-board (3 months).
(2) After half a year of overdue, it is dried after being dried at 125 °C and SHr.
(3) Overdue for more than one year of scrapping.
High Tg sheets are easy to absorb moisture. If they are packed in polyethylene plastic, the moisture absorption will double after 3 months. The actual test by the author shows that the 7-month moisture absorption is about 4.3%, but it can reach 12.3% in 4 and a half months. Therefore, for high T and board, it must be packaged in aluminum foil and limited to production within 3 months! A major change in the assembly process.