| Sumario: | Printed Circuit Boards (PCB’s) are an important component for any electronic device,
they can be found in cell phones, computers, tablets, televisions, radios, remote
controls, among others. In automotive industry, PCB’s are incorporated into the
board containing tachometers, water, oil, gasoline levels and all signals displayed on
the board. PCB’s contain electronic components such as: electric motors, resistors,
capacitors, micro-controllers and LED’s, just to name a few. To join them, a solder
paste of tin, silver and copper alloy (Sn-Ag-Cu) is applied on the PCB surface; then
the PCB is processed into a reflow oven at temperature range of 24oC-250oC allowing
the solder paste to flow and join the electronic components with the PCB, an
operation called “reflow process”.
In 2006 the Restriction of Hazardous Substances (RoHS) prohibited the use of
lead (Pb) in solder paste, which has a melting temperature around 180oC, whereas
lead-free solder pastes have a melting point around 230oC, 30oC-40oC higher than
Pb solder paste, this has generated PCB thermo mechanical problems due to the
temperature increase at which the material is exposed. Typical problems are related
with deformations of the PCB in the reflow process, such as “warpage”, which is
a deformation along the “z” axis and is accompanied by other phenomena called
“bow” and “twist’, “bow” is characterized by a curvature of cylindrical shape on
both sides of the PCB whereas “twist” is characterized by the elevation of the corners.
“Warpage”, “bow” and “twist” affect subsequent processes such as: assembling
engines, micro-controllers, improper electrical tests, false contacts, bending of electronic
components and fractures at the interphase between electronic component and
the PCB.
The present research work studies the relation between material thermal properties,
PCB configurations and reflow conditions with “warpage” “bow” and “twist”
during the reflow process by the thermal characterization of base materials, deformations
measurements and temperature reflow profiles. Thermal properties obtained
were: glass transition temperature (Tg), decomposition temperature (Td), Coefficient of thermal expansion (CTE), time to delamination and %water absorption,
deformations measurements were obtained on 30 PCB’s after exposure. Temperature
profiles were obtained by placing thermocouples on the PCB.
Our results suggest that there is a discrepancy between the thermal properties
obtained experimentally and data sheet provided by the supplier. PCB “bow” and
“twist” data obtained exceeds the values established by the IPC-2221B standard and
the temperature profiles met the requirements of the quality control in the company.
It is found that there is a mismatch between temperature profiles, it is speculated a
relationship with preferential deformations during reflow process.
The present work contribution consists in a whole study of PCB thermo-mechanical
performance during reflow process considering material thermal properties, reflow
conditions and the influence on PCB deformations.
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