The development trend of the electronic information industry has higher and higher requirements for the PCBA assembly process, and the reliability and quality of the electronic product is mainly determined by the reliability and quality level of the PCBA. In the process practice and the failure analysis of the PCBA, the author It is found that the residue on PCBA has a great influence on the reliability level of PCBA, but it is not paid attention to by people, especially process engineers or related quality control personnel, although this situation is currently changing. However, because the reliability of PCBA often occurs after users have used it for a period of time, and the technical means of various manufacturers are limited, it is impossible to understand and evaluate the effect of residues on PCBA because these failures cannot be linked to the existence of residues. The impact of reliability. Among the residues, inorganic residues will reduce the insulation resistance, increase the leakage current between solder joints or wires, and will corrode the metal surface under humid air conditions, and organic residues (such as rosin, grease, etc.) will form insulation This will hinder the electrical contact between the contact surfaces of connectors, switches, relays, etc., and these effects will increase with changes in environmental conditions and time extension, causing poor contact or even open circuit failure.

Therefore, for the reliability and quality of PCBA, the presence of residues must be strictly controlled, and these contaminants must be thoroughly removed when necessary. This article first introduces the source and analysis method of the residue, then analyzes the impact of the residue on the reliability of PCBA in detail and proposes the control measures and methods for the residue.

 

 

1. Types and sources of residues

The residues on PCBA mainly come from the assembly process, especially the welding process. For example, the flux residues used, the by-products of the reaction between the flux and the solder, adhesives, lubricating oil and other residues. Some other sources have relatively low potential hazards, such as pollutants and perspiration caused by the production and transportation of components and PCBs themselves. These residues can generally be divided into three categories. One type is non-polar residues, mainly including rosin, resin, glue, lubricants, etc. These residues can only be removed with non-polar solvents for cleaning. The second category is polar residues, also called ionic residues, which mainly include active substances in the flux, such as halogen ions, and salts produced by various reactions. These residues need to be removed well, and polar residues must be used. Solvents, such as water, methanol, etc. There is another type of residues that are weakly polar residues, which mainly include organic acids and alkalis from the flux. To obtain good results in the removal of these substances, a composite solvent must be used. The basic categories of residues are specifically introduced below.

1.1 Residues of rosin flux

The flux containing rosin or modified resin is mainly composed of non-polar rosin resin and a small amount of halide, organic acid, and organic solvent carrier. The organic solvent will be volatilized and removed due to high temperature during the process. Active substances such as halide organic acids (such as adipic acid) mainly remove the oxide layer on the welded surface and improve the welding effect. However, in welding, the complex chemical reaction process changes the structure of the residue. The product can be unreacted rosin, polymerized rosin, decomposed active agent and active agent such as halide, metal salt produced by the reaction with tin-lead, unchanged rosin and active agent are easier to remove, but it has a potentially harmful reaction It is difficult to remove materials.

1.2 Organic acid flux residue

Organic acid flux (OR) generally refers to the flux in which the solid part of the flux is mainly organic acid. The residues of this kind of flux are mainly unreacted organic acids, such as oxalic acid, succinic acid, etc. and their metals. Salt. At present, most of the so-called colorless and no-clean fluxes on the market are of this type. They are mainly composed of multiple organic acids, including halogen-free ions at room temperature, and compounds that can produce halogen ions at high temperatures, sometimes including very small amounts. Among these residues, the most difficult to remove are the salts formed by organic acids and solders. They have strong adsorption properties and extremely poor solubility. When the PCBA assembly process uses water-soluble flux, a larger amount of these residues and halide salts will be generated, but due to timely water-based cleaning, such residues can be greatly reduced.

1.3 White residue

White residue is a common contaminant on PCBA, which is generally found after PCBA has been cleaned or assembled for a period of time. PCB See PCBA Many aspects of the manufacturing process can cause white residue (see Figure 1).

 

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Figure 1 Typical white residue on the surface of PCBA

The white contaminants of PCBA are generally by-products of flux. However, poor quality of PCBs, such as the adhesion of solder mask, will increase the chance of white residues. Common white residues are polymerized rosin, unreacted activator and the reaction product of flux and solder, lead chloride or bromide, etc. These substances expand in volume after absorbing moisture, and some substances also undergo hydration reaction with water. White The residue is becoming more and more obvious. It is extremely difficult to remove these residues by adsorption on the PCB. Natural rosin is prone to a large number of polymerization reactions during the welding process. If overheating or high temperature for a long time, the problem will be more serious. The infrared spectrum analysis results of the rosin and residue on the PCB surface before and after the soldering process confirm this process.

 

 

 1.4 Adhesive and oil pollution

In the assembly process of PCBA, some yellow glue and red glue are often used. These glues are used to fix components. However, due to process inspection, the electrical connection parts are often contaminated. In addition, the residue torn off the pad protection tape will seriously affect the electrical connection performance. In addition, some components, such as small potentiometers are often coated with too much lubricating oil, which will also pollute the PCBA board. Such pollution residues are often insulated, which mainly affect the electrical connection performance, and generally will not cause corrosion, leakage, etc. Failure problem.

2 Analysis method of residue

According to the latest EIA/IPC J-STD-001C (Technical Requirements for Electrical and Electronic Welding, 2000 Edition), the requirements for the residual amount of the surface of each accessory before PCBA assembly and the residual amount of PCBA after welding must first To ensure the solderability requirements of various accessories, the bare PCB must have a clean appearance, and the equivalent ion residue must be less than 1.56mgNaCl/cm2 (solvent extraction method). For PCBA, in addition to the ion residual amount not greater than 1.56mgNaCl/cm2, the requirements for the residual amount of rosin resin flux and the requirements for appearance are also specified. See Table 1 for details.

Table 1 Residues and cleanliness requirements and analysis methods of electronic assembly process

 

Components and PCB

PCBA

Analytical method

Exterior

Clean without affecting

Solderability

Clean, no dirt, tin dross

Tin beads and unfixed metals

No particles allowed

Visual inspection with microscope or magnifying glass

Ion residue

<1.56mgNaCl eq./cm2

<1.56mgNaCl eq./cm2

Or agreed by both parties

IPC-TM-650

2.3.25

2.3.28

Rosin resin residue

Thing

Clean, no residue

Thing

Class1<200mg/cm2

Class2<100mg/cm2

Class3<40mg/cm2

IPC-TM-650

2.3.37

SIR

 

Customize with reference to relevant standards

EIA/IPC J-STD-001C

Organic residue identification

do not

Clean, no residue

The manufacturer and the user agree

Restricted substance.

IPC-TM-6502.3.39

 

Analysis and detection methods for residues or cleanliness on PCBA mainly include appearance inspection, ionic residues, rosin or resin residues, and the identification of other organic pollutants. The main testing standards are shown in Table 1, which are briefly introduced below.

2.1 Visual inspection can generally be done by visual inspection. If necessary, use a magnifying glass or microscope to observe solid residues. It is usually required that the PCBA surface must be as clean as possible without obvious residues, but this is a qualitative indicator, usually based on the user’s Requirement as the goal, make your own inspection judgment standards, and use the magnifying glass for inspection.

2.2 Ionic residue analysis method Ionic residue usually comes from the active material of the flux, such as halide ions, acid radical ions, and metal ions produced by corrosion. The result is expressed by the equivalent number of sodium chloride (NaCl) per unit area. That is, the total amount of these ionic residues (including only those that can be dissolved in the solvent) is equivalent to the amount of NaCl, which is not necessarily present on the surface of PCBA or only NaCl is present. Tests generally use IPC methods (IPC-TM-610.2.3.25), which include manual extraction, dynamic (instrument) extraction, and static (instrument) extraction. Rinse (or extraction) solvent (usually 75±2%V/V isopropanol and DI water, or 50±2%V/V isopropanol and DI water, the latter is less used) to rinse the PCBA surface to remove ions The substance is dissolved in the solvent, carefully collect the solvent, and then measure its electrical conductivity (resistivity), if the instrument is used, it will be done automatically. Use the relationship between the ion concentration and the change in resistivity to obtain the total amount of ions, and then divide it by the surface area of ​​the PCBA to obtain the ion contamination value per unit area of ​​the PCBA. (MgNaCl/cm2) This kind of test is generally prepared with a standard NaCl solution in advance, and then calibrated to obtain a standard curve.

The resistivity of the mixture used must be greater than 6MΩ.cm. When washing and extracting PCBA residues, the amount of mixture used is generally 1.5ml/cm2, and the maximum is no more than 10ml/cm2. During operation, the volume of solvent collected is not critical, but the total volume used for cleaning It must be strictly recorded. At the same time, since the temperature has a great influence on the cleaning effect and the measurement of resistivity, it is necessary to explain the temperature conditions during the measurement. In addition, the measurement and calculation of PCBA and PCBA area are unified as follows: bare board PCB with no components inserted: length × width × 2, and PCBA due to components, the surface area of ​​the PCBA increases to 50%, but generally increases The amount is 10% of the original surface.

Therefore, the PCBA results should also indicate a. solvent composition, b. solvent volume used by static solvent or flow rate used by dynamic solvent, and C. test temperature. D. Calibration situation, E. Area (calculation method) F. Test time, G. Instrument used. In addition, the instruments used in static and dynamic methods include IonChaser, Ionognagh and OmegaMeter. The numbers of them and the results obtained by the manual extraction method are 3.2/2.0/1.4 respectively. Therefore, the measurement results must be indicated to the instrument and equipment. In addition, the dynamic method can record the change of resistivity during the extraction of the residue by the instrument and the instrument, which is useful for understanding the residue. Have a clear understanding of the dissolution process of the substance and help to choose the cleaning process.

In many cases, due to the residue of various ions, the impact on the reliability of PCBA is not the same. It is not only necessary to know the total amount or equivalent of the residue ions is not enough. When we also need to know the halide ions or other ions that have a greater impact, we use another method to analyze, that is, according to IPC-TM-610.2.3.28, use an ion chromatograph to clean the mixed solvent (80℃, 1h). The ions are measured and analyzed one by one. Then, it is converted into the residual amount of ions per unit surface. Table 2 shows the requirements for the cleanliness of the ions on the surface of the air conditioner main board by a famous electrical company in the United States.