Abstract:
The OSP (organic solderability preservative) in the current market is genially easy to precipitate crystals in the production and storage, it make the equipment be polluted. We prepared an OSP with better low temperature stability by adding a stabilizer, and compared with ordinary OSP. The results showed that the storage temperature of low temperature-resistant OSP is lower than ordinary OSP 10-15 . Its f ilm thickness decreased, and the film thickness increased from 0.17 m to 0.39 m as follow as the content of OSP main agent from 6g/L to 10g/L, the preliminary reason that against low temperature is because the stabilizer enhances the solubility of the main agent. The relationship between OSP f ilm's hydrophobic and the content of each ion in composition was discussed. The film ability of resistant temperature and solderability is well, while the temperature up to 310 , there are still good on the tin.
Introduction
In PCB industry OSP (Organic Solderability Preservatives) process is a favored treatment technology on PCB (printed circuit board) copper surface. In the current OSP coating process has been developed to the sixth generation, the film on copper surface has high temperature resistance, compatible with a variety of flux, and has broad market prospects. The advantage of OSP coating process is easy operation, mild conditions, without organic solvents, low cost, and the waste water treatment is simple. The outstanding characteristic of the sixth generation OSP is having high temperature resistance , but one of the problems of the OSP composition is easy to precipitate in process of storage and production, causing pollution to equipment and great inconvenience for the production , also increasing cost. So preparing OSP with low temperature stability is very important to solve these problems.
At present, research about the low temperature stability of OSP hasn't been reported, so low temperature-resistant OSP compositions were prepared in this paper, the performance, such as temperature resistance , solderability , even the influence of improving the OSP performance on production and economic efficiency were researched. The paper mentioned the influencing factor of OSP film's hydrophobic, it's a basis for more deeply discussed the film performance.
1 Experimental
1.1 Composition of OSP compositions
The following OSP compositions is used to protect copper surfaces, OSP A composition (Table 1) is ordinary OSP and OSP B composition (Table 2) is named by low temperature-resistant OSP, because it can be stored at low temperature. The difference is OSP B was added 4mL stabilizer. The percentage content of compositions at 100mL OSP solution are also showed in Table 1 and 2.
1.2 OSP composition characteristics
The OSP composition characteristics are shown in Table 3.
1.3 OSP film processing conditions
A test piece of PCB is immersed in the OSP composition at the temperature of 38-42 for 30-70 seconds, whose pH is adjusted to 2.8-3.5 with ammonia water (5%), total acidity is 90-130%, concentration is 90-120%. Thickness of OSP film on a single PCB is 0.2-0.4 m and that on double-sided multilayer PCB is 0.25-0.4 m.
1.4 OSP application process
1.5 Determination of OSP film thickness and hydrophobic
The area of a piece of PCB copper with OSP film was measured, the surface may be rinsed with water, after cleaning thereon may be dried. The HCl solution in 5% v/v is applied as reference for immersing the test piece for 5 minutes, test the absorbance values at wavelength 270nm with a UV spectrophotometer, calculate f ilm thickness by the formula follow as, thickness ( m) = 0.253 A Y/X, where A is the absorbance, X is the area of test piece, Y is the volume of dissolved liquid. The hydrophobic is determined by the liquid film contraction time on layer surface. The liquid film contraction time is the time from taking out PCB having OSP film from OSP solution, covering liquid film, to the liquid f ilm contracting into a drop and dropping. The time is more shorter, the hydrophobic is more better, furthermore, the film compactness is also good.
1.6 Test for temperature resistance and solderability
Temperature resistance test method is that the test piece treated by OSP is baked in the oven at 310 for 60 seconds, and removed to react with air for 12 hours, repeating three times. If the color of piece is changed, it's thought that the piece could be oxided and the OSP film on the piece hasn't good temperature resistance. The solderability test method is that the test piece smeared flux is immersed in tin stove at 310 for 3 seconds, if the piece area adhered by the tin is more than 90%, it's thought that the OSP film on the piece has good solderability.
2 Results and discussion
2.1 Stability of OSP composition at different temperature
At different temperature, the OSP A composition and OSP B composition were aged for three days. It is apparent from Table 4 that precipitation and turbidity caused by crystallization of the main agent when pH is adjusted with ammonia to optimum. The main solvent of OSP A composition is the organic acid, which have two or fewer carbon atoms and is characterized by high volatility and low boiling point, so, over time, many of the agent crystallize. Meanwhile, the solubility of the main agent which is phenylimidazole decrease with the temperature, as Table 4 showing, the main agent have turbid when the temperature get to 25 for 48 hours. However, the OSP B composition with the stabilizer is clear at 5 for 48 hours. The stabilizer characterized low volatility and high boiling point greatly increase low temperature solubility of the main agent. The OSP B composition storage
temperature is lower 15 than OSP A composition, the result completely meet the requirements of low temperature production.
2.2 Critical pH for the crystallization of OSP composition at different content of main agent
OSP A and B composition are prepared containing different content of main agent, ammonia water may be added to adjust the critical pH that is two kinds of OSP composition start to crystallize at room temperature.
On the Fig 2, the above area of the line is crystallization f ield and the below area of the line is no-crystallization f ield. Following the increase of the content of the main agent, the critical pH get smaller. The pH that OSP A composition becomes turbid is 3.3 at room temperature when the content of main agent get to 10g/L, however, at the same condition pH is 3.5 to the OSP B composition. The result shows that the main agent solubility of low temperature-resistant OSP is better than that of ordinary OSP, the composition can keep clear when pH was adjusted within the range of 2.8-3.5 at the high main agent concentration. The reason is that the stabilizer increased the solubility of the main agent in composition at high pH value, so that the low temperature-resistant OSP composition's critical pH is higher than that of ordinary OSP.
2.3 The thickness of film deposited from the OSP composition at different content of main agent
OSP A and B composition are prepared containing different content of main agent, ammonia water may be added to adjust the pH to from film on the copper surface. OSP A and B composition were used to deposit OSP f ilms over copper surface. The f ilm thicknesses achieved are shown in Fig 3.
From the principle of OSP film formation we can see that the more the content of the main agent in OSP composition, the more easily react with the metal substrate, the faster the f ilm growth and the greater the f ilm thickness is. In the Fig 3, while the experimental results also verify this conclusion, but in the Fig 2, the critical pH of OSP in different content of the main agent shows that the more the content of the main agent, the more easily precipitate crystals. Therefore, we can reduce the formation time through increasing the content of the main agent in further to improve production eff iciency. Also, there must be no crystals within the prescribed range o f pH a t the high concentration of the main agent in OSP composition, and the low temperature-resistant OSP can meet this requirement.
From Fig 3, we can see that, at the same content of the main agent, the thickness of OSP B film is lighter than A, but still remain at 0.2 m or more. It can be considered that the addition of stabilizers enhanced the solubility of main agent and the main agent of good solubility when f ilm-forming is not easy to react with the metal surface, so the f ilm is lighter. It is useful for saving phenylimidazole to decrease the film thickness of OSP within the range of technological requirements and is good for economic eff iciency in the actual production process. In short, low temperature-resistant OSP can quickly form lighter f ilm and no crystal precipitated at the high content of the main agent while improves production and economic eff iciency greatly.
2.4 OSP film's hydrophobic
The influencing factor of OSP f ilm hydrophobic is the concentration of ions (Table 5), when the weight rate of and and in 100mL OSP composition is 140:0.4:1.3, the two kinds of
OSP film all have good hydrophobic and smooth surface, it's meet the industrial requirements. The result is that hydrophobic is closely related to the layer compactness, good hydrophobic lead to good compactness. The stabilizer added in low temperature-resistant OSP hasn't effect on the weight rate of and and . So, the low temperature-resistant OSP layer of good hydrophobic and compactness also have ability of resistant fingerprint and besmirch.
2.5 Temperature resistance and solderability
The pieces treated with A OSP and B are numbered as A1 and B1 separately. The test results suggest that the OSP A and B film both can withstand heat shock at 310 for three
times (Table 6), so two kinds of f ilm have good temperature resistance. The f ilm can be well compatible with the flux and the piece area adhered by the tin can get more than 95% on the high temperature solderability test, in short, the solderability is good. The conclusion is that the film of low-temperature stability of organic solderability preservative containing stabilizer still has good temperature resistance and solderability, the stabilizer hasn't effect on the f ilm's temperature resistance and solderability, they all can keep well.
3 Conclusions
Low-temperature stability of organic solderability preservative comprises organicacids , phenylimidazole, zinc and copper ions, stabilizer. The advantage is that it can be stored and used at low temperature (even 5 ), especially solve the industrial problems that the OSP composition is easy to crystallize in low-temperature production environments. It can be known from researching the thickness at different content of main agent, the more the content of the main agent in OSP composition, the faster the f ilm growth. We can reduce the formation time through increasing the content of the main agent in further to improve production eff iciency, meanwhile, there must be no crystals within the prescribed range of pH of OSP composition, and i t is us e fu l for saving phenylimidazole that the deposited f ilm is lighter and increased the economic eff iciency. We can preliminary judge that against low temperature is because the stabilizer agent enhances the solubility of the main agent.
The low temperature-resistant OSP f ilm on the copper surface has good hydrophobic and compactness. It's think that the influencing factor of OSP film hydrophobic is the concentration of ions. The low temperature-resistant OSP f ilm can withstand heat shock at 310 for three times and have no color change, also i t have good solderability and meet the industrial requirement completely.
