The study of heat dissipation process during PCBs drilling
Instrument production technology
2. Moscow Aviation Institute (National Research University), 4, Volokolamskoe shosse, Moscow, А-80, GSP-3, 125993, Russia
It is well known, that approximately 30% of failures occur in PCBs interconnections. Drilling is still the most popular process for producing a hole. The main surface defects of the holes appeared during PCB dielectric base drilling are as follows:
— ribs formation at the entry and outlet of the drilled hole.
The occurrence of these defects is associated primarily with the drill bit heating during drilling. The accuracy of drilling is highly affected by the quality of the PCB, which, in turn, depends on the quality of the alignment layers and the parameters of the material [1-3].
It should be noted that the defects caused by drilling, could lead not only to failures, but also increase the products cost due to the increased cost of subsequent phases , as they require additional operations introduction to PCB manufacturing process.
While drilling heat is generated in the hole by frictional force on the back surface of еру drill. Cooling of drill bit takes place during the tool transition from hole to hole. The drill will be considered as isotropic heat conducting rod.
Let us consider the following practical conditions of a drilling process, namely:
— length to drill diameter ratio is greater than 10;
— heat transfer is significantly complicated;
— heat transfer, caused by heat exchange between the drill and the material of the dielectric substrate is even more complicated.
The equation of heat dissipation in the drilling cycle of heating
Here, T — temperature of the element of the drill; — time; µ — friction coefficient on the drill back face; N — cutting force; v— velocity of the application point of the force N; λ —thermal conductivity coefficient; s — drill perimeter; α — heat transfer coefficient.
Equation for the top of the drill takes the form,
T* — temperature at which cooling starts while idling;
Tc — ambient temperature;
b — coefficient of heat exchange with environment.
The above relations allow make preliminary calculations of the drill temperature during the process of g holes drillin in PCBs and, thus, to determine the number of drilling cycles that can be performed without the risk of defecting holes.
Keywords:drilling, heat dissipation, printed circuit boards
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