label: printed board,circuit board material,PCB feature
IPC-1601 provides users with guidance on how to protect printed boards from contamination, physical damage, solderability degradation, electrostatic discharge and moisture uptake. The document covers all phases of production, from the manufacture of the bare printed board, through delivery, receiving, stocking, and soldering. It also gives needed attention to helping users with moisture concerns. The following information provides guidance on establishing recommended moisture levels and baking profiles for moisture removal, and covers the impact of baking on printed board solderability. Moisture absorbed in printed board laminates expands at soldering temperatures, and in some cases the resulting vapor pressure can cause internal delamination or excessive strain on plated-hole walls and other structures. This is especially challenging with the higher temperatures used for lead-free soldering. The costs associated with having too much moisture in a board can be significant. System manufacturers note that if moisture causes problems and ruins boards after parts have been installed, failures can have major repercussions.
•The transition to lead-free soldering of printed circuit boards has resulted in a peak reflow and wave soldering temperature increase of 30-40°C for a longer period of time during assembly compared to leaded solder technology.
•These high temperature exposures associated with lead-free soldering assembly conditions result in variations in the circuit board material properties, therefore creating a shift in the expected reliability of the board. This may increase the effect of the moisture that may be absorbed by the base materials; the moisture reduces glass transition temperature of the base material which in turn increases stresses on PCB features and can cause blistering or inner layer delamination.
•IPC standard highly recommends that, baking of the boards prior to assembly is needed to remove the moisture that can be absorbed into PCB; the baking oven is to be set below the maximum operating temperature and glass transition temperature but above 100° C; the baking time should be selected based on the boards features, such as PCB complexity, overall thickness, and PCB finish; it is also recommended that baking be performed in a forced air recirculating oven; it is important to insure proper venting and cleanliness of the oven and sufficient gaps between boards; the boards should never be high stacked inside of the baking oven.
