PCB Storage Management and conservation of PCBs for the purpose of maintaining the characteristics and preventing possible deterioration of performance

Hygroscopy is one of the most common properties of laminates used for the PCB production, that is the characteristic of absorbing humidity, that cause external delamination of the solder mask, also called “blanching”, and, in the presence of a multi circuit layers, other internal delamination, difficult to locate, which cause the separations and interruptions of the various circuit layers. (see Fig. 1)

For example, FR-4 (Flame Retardant 4), one of the most common material used for pcb manufacturing, after 50 hours from the end of the its production may reach a dangerous level of saturation for the next welding phases.

The IPC-1601A standard of IPC (Institude of Printed Circuits – Association Connecting Electronics Industries – www.ipc.org), sets the basis to guarantee proper management. IPC-1601A suggest behaviour regarding handling, the IPC-1601A suggest packaging methods, external contamination protections, physical damages, electrostatic charges, environmental conditions storage, moisture and degradation of weldability.
For a correct PCBs storage, pre- and post-production, is essential that all IPC-1601A prescriptions will be taken.

User can require to its supplier all proper storage requirements such as the specific label HIC (Humidity Indicator Card) should include in the package, for detect and show the moisture percentual, and other specifics precautions and tools need to prevent moisture absorption.
Any type of component has a value measured in TG ((Glass Transition Temperature). The higher the TG value, the more hygroscopic the tested component. Some materials such as polyamide based Kapton or CEM-3 (Composite Exposy Materials) have a greater propensity to absorb moisture.

In some cases, even flexible or rigid flexible materials, after assembling the components, may have different swollen areas (popcorn effect) due to the delamination process caused by the presence of moisture.

The possible consequences
The PCB inner layers delamination can cause barrel crack that can get to break the circuit or malfunctions of the board.

To limit the problems caused by moisture, in particular for multilayer PCBs, it is strongly advisable to make a “baking” cycle. This treatment consists to the passage of PCBs in special ovens before packaging for shipping or before placing them on the assembly lines in order to reduce moisture. Baking’s temperature and time depending on the finishing surface used and other parameters of the PCB.

IPC 1601 also provides parameters relating to the maximum moisture content supported by aa PCB. If moisture quantity than 0.1%, the risk of delamination is high; below the 0.1% moisture is defined as the optimum quantity.
To calculate the percentage of moisture, it is necessary to weigh the circuit with a special laboratory scale before baking.
At the end of baking process, the circuit is re-weighted determining, by weight difference, the percentage of moisture eliminated. Sometimes, to obtain an excellent result, it is necessary to repeat the procedure several times.
When introduce in the market the new Pb Free processes, the laminate manufacturers developed laminates which -in addition to being RoHS conform- offer better performance even when they were subjected to higher reflow temperatures in the SMT (Surface Mounting Technology) lines.
Most of these “new laminates” are however more hygroscopic, which means that to prevent their moisture absorption, it is important to check the time of storage (shelf life), take care of the packaging type, keep the environment controlled, because moisture must be less than 0.1%.
It is good practice that the PCB are stored in closed rooms (dry room – dry box) with Relative Humidity (RH) RH <= 10% and kept at a temperature between 15 and 25 ° C.
The Dry-box or Dry-pack, sealed under vacuum, guarantees a RH equal to or less than 5% for 12 months. Once the package has been opened, the floor life counting of the circuits begins.
In any case, even in the presence of vacuum packaging, it is however not recommended to store the circuits for a time longer than six months.
The baking process must be carried out, using a static oven, better with air circulation, positioning the circuits in stacks consisting of no more than 5 circuits.
The best condition is to position the circuits vertically, distancing them by a few centimetres.
It is important to do the baking just before to use the PCB for assembly. This because:
1.once moisture is removed (baking), the FR4 tends to reabsorb it faster (“sponge” effect)
2.baking accelerates the aging of the surface finishing
For this reason, it is strongly recommended to recondition ‘only’ the circuits that are necessary for the production in progress.

For more Information:
E-mail: customer.care@tecnometal.net