● Positive factors of dry film
● Negative factors of dry film
Traditional printed circuit boards use rarely soldermask, however, the SMT circuit boards generally will use solder mask. Solder mask is a polyethylene material, mainly divided into two types, dry film and wet film. This article will mainly introduce the positive and negative factors of dry film.
positive and negative factors of dry film
When the non-permanent solder mask is soldered in wave, wave will go through tooling holes in the circuit board to the non-welding surface. Therefore, before the insertion of components, the mounting holes and gold finger should be covered up with a non-permanent solder mask and then be dissolved away during the cleaning.
Permanent solder mask is an integral part of the PCB board. It prevents the bridges from connecting together while soldering in wave and also protect the wiring from mechanical damage or chemical corrosion. Permanent solder mask is divided into dry and wet film. Dry film is water-based or solvent-based.
Dry film solder mask graphics has high resolution, suitable for high density wiring boards, and can be accurately aligned. So it will not flow into the through holes in the circuit boards. When the circuit board is tested in needle bed, the vacuum should be used to suck the board to locate. What’s more, dry film is not easy to be polluted, so that it will not affect the reliability of welding.
There are some negative factors using dry film:
On the surface of PCB boards there are pads, lines, so the surface is not smooth, and the dry film solder mask has no liquidity. So, between the dry film and the surface of the circuit board there may be the gas left. The gas gets expanded when heated, which may cause dry film rupture phenomena.
Dry film is relatively thick, generally the thickness is 0.08 ~ 0.1mm (3 ~ 4mil). The dry film covering on the surface of circuit boards, may cause the offset phenomenon.
Curing conditions of dry film solder mask is very strict, if the curing temperature is too low or the curing time is too short, then the curing is insufficient. It will be affected by the solvent while cleaning. If the curing is too brittle, it may crack when exposed to heat stress. What’s more, dry film solder resist has poor resistance to thermal shock, easy to get cracked in high temperature. The price of dry film is high than wet film.
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