Causes and improvement methods of photosensitive soldering white oil yellowing on printed circuit board

This paper mainly introduces the causes and improvement methods of the yellowing of photosensitive soldering white oil from the properties and characteristics of the ink itself.


Printed circuit board photosensitive resistance welding white oil belongs to the printed circuit board photosensitive solder resist ink, and there are many common points, photosensitive solder resist ink are made by resin, monomer, initiator, diluent, filler, pigments, additives, the main difference is embodied in the use of different raw materials, this article is from the resin, initiator, pigments and additives to analyse the causes of yellowing photosensitive resistance welding shortening and improving methods.

First, resin

First discuss is resin, known as resin, as A skeleton of A printing ink is indispensable, for the photosensitive resistance welding used white oil resin is particularly important, photosensitive solder resist ink (for example, green oil, black oil, blue oil) resin used in many neighbors A modified phenolic epoxy resin and bisphenol A epoxy resin, and introduced first grafted acrylic photosensitive groups double bond, then introducing grafting anhydride alkali enhancement carboxyl functional groups. The resin of this structure has a benzene ring structure in the main body, and the benzene ring structure is easy to generate free radicals in ultraviolet radiation and generate conjugated system with chromophon group, which is easy to turn yellow.

The structure of the resin photosensitive resistance welding shortening yellowing is serious, can't meet the needs of customers, so at present, a lot of photosensitive resistance welding shortening of resin for acrylic resin, usually with epoxy groups of acrylate copolymer, then grafted acrylic introducing photosensitive groups double bond, the grafting anhydride introduced basic developing carboxyl functional groups. 

This acrylic structure of alkali development resin because it is aliphatic acrylic epoxy resin, uv or high temperature is not easy to produce hair color group, solid anti yellowing effect should be better.

Second, initiator 

Since this paper is to discuss the photosensitive soldering white oil, photoinitiator is necessary in the system. The initiator photosensitive solder resist ink system for isopropyl sulfur impurity anthracene ketone (ITX), 2 - methyl - 1 - (4 - methyl sulfonium phenyl) - 2 - morpholine - 1 - acetone (907), 2 - amino - 1-2 - dimethyl benzyl - (4 - phenyl) morpholine butanone (369), the initiator system caused by active high light, basic can satisfy the use of most of the photosensitive solder resist ink, but this kind of initiator is containing sulfur, nitrogen and other impurity atoms, after uv irradiation is also easy to produce hair base group and yellowing, so this kind of initiator system is not suitable for the use of photosensitive resistance welding shortening.

Photosensitive resistance welding shortening more initiator system currently in use is not easy to produce more yellowing of the product, such as 1 - hydroxy cyclohexyl phenyl ketone (HCPK), 2 minus 2 - trimethylbenzene formyl - diphenyl phosphine oxide (TPO) and phenyl double (2 minus 2 - trimethylbenzene formyl) phosphine oxide (819), especially the TPO, it is a kind of free radical type light initiator, suitable for low yellowing and white system use, have light bleaching effect.

Third, toner

Photosensitive solder resist ink using very little amount of toner are generally phthalein cyanine, phthalein green organic pigments, such as adding quantity generally in of a few to a few parts per thousand, and photosensitive resistance welding shortening use toner is mainly titanium pigment, content in 20-40%, at present the mainstream market has sharp titanium type and rutile type titanium dioxide, titanium sharp type titanium dioxide in light scattering, whiteness and hiding power are better, but poor weather resistance to some, titanium and sharp type titanium dioxide itself the side effects of the aging of the polymer and the catalytic cracking; Rutile titanium dioxide because the surface is processed, so in the weather resistance is better, a lot of photosensitive soldering white oil is made of lighting board (such as car lamp board, LED lamp board), long-term in the light irradiation, the weather resistance requirements are higher, so the preferred rutile titanium dioxide. At present, the market routine use of titanium dioxide such as dupont R706, R960, R103, ishihara R680 and so on. 

In addition, in order to make people feel whiter, photosensitive soldering white oil will generally add some bluish phase toner into the system. The traditional bluish phase phthalocyanin is vulnerable to ultraviolet light or high temperature aging, which is generally not used. Commonly used is group of green, because group of green is an inorganic substance, is not easy to dissolve at high temperature is not easy to crack and itself is not easy to yellow, in addition, group of green can eliminate photosensitive soldering white oil in the yellow light, play the white effect.

Four, additives

Photosensitive resistance welding shortening system using resin in high temperature or long term aging process occurs under ultraviolet light, the aging is polymer pyrolysis process of active free radicals continuous reaction, aging and cracking of yellowing, so photosensitive resistance welding shortening in addition to add flow ping agent, dispersing agent, defoamer, and other general agent, specific resistance to yellowing of the need to add additives, antioxidant used the market at present basically has the following three.

4.1 amine antioxidants

Amine antioxidants mechanism is by capturing the oxygen free radicals to prevent or inhibit chain and chain reaction, so as to terminate the radical chain reaction, reach the purpose of preventing oxidation, in short, anti-aging effect such as para-phenylene diamine, phenothiazine, easy to oxidation and discoloration, but itself in photosensitive resistance welding white oil is not easy to add.

4.2 phenolic antioxidants

Steric hindrance phenolic antioxidants are phenolic compounds with spatially blocked structures, such as basf Irganox1010, Irganox1076 and sumitomo ga-80.

The mechanism of action of steric hindrance phenolic antioxidants is mainly to capture the active free radical to generate the inactive free radical, so as to terminate the chain reaction. The mechanism of action is as follows:

PCB soldermask

This kind of antioxidant is a general antioxidant of photosensitive soldering white oil, which is generally not used alone and has the best effect in combination with auxiliary antioxidant.

4.3 auxiliary antioxidants 

Auxiliary antioxidants are mainly synergistic with steric phenolic antioxidants, which are mainly phosphite esters and thioesters. Their role is mainly to destroy the hydroperoxide produced in the aging process of the polymer and timely prevent the self-oxidation process of the polymer. At present, there are three main phosphite esters (commonly known as antioxidant 168) in the market.

In general, sterophenol antioxidant and auxiliary antioxidant are used together with photosensitive soldering white oil. There are some products in the market, which are mainly used in plastic high-temperature injection molding, while the experimental results shall prevail in the application of photosensitive soldering white oil.

In the development of photosensitive blocking white oil, we pay attention to the above points, pay attention to the selection and matching of raw materials, and finally through the optimization of the formula, we can quickly develop excellent resistance to yellowing photosensitive blocking white oil.