Quality inspection of incoming materials for SMT chip processing

2020-12-15 14:53:36

Quality inspection of incoming materials for SMT chip processing


1. Tasks and methods of raw material quality inspection


Task: Four aspects: raw material quality judgment, quality problem prevention, quality information feedback and quality problem arbitration


Methods: three categories: sensory testing, appliance testing, and trial testing


Quality judgment: Refers to judging the qualified degree or quality level of raw material quality through testing in accordance with relevant quality requirements and specifications


Quality problem prevention: Refers to ensuring that unqualified raw materials are not put into use through quality inspection, so as to prevent the resulting quality problems.


Quality information feedback: Refers to the feedback of the quality problems of raw materials to relevant departments or cooperative enterprises through quality inspection, to find out the causes of quality problems in time, and to provide a basis for improving quality.


Quality issue arbitration: When the raw material supplier and the recipient have disagreements or disputes about the quality issue, scientific quality testing and evaluation methods are used to determine the cause and responsibility of the quality issue.


2. SMT assembly materials include components, PCB, solder paste, flux, adhesive, cleaning agent and other assembly process materials.


3. The main inspection items of components: solderability, lead coplanarity, performance


4. Solderability of SMT components: mainly refers to the solderability of solder ends or pins.


Influencing factors: Oxidation or contamination of the solder ends or pin surfaces of components.


Component solderability testing methods: there are many, the more commonly used are-solder bath dip method, solder ball method, wetting weighing method


5. Welding bath infiltration method


The sample is immersed in the flux and then taken out, the excess flux is removed and then immersed in the molten solder tank about twice the actual production soldering time, and then taken out, and then visually evaluated: all samples to be tested should show a continuous solder coverage, or At least the solder covering area of each sample reaches 95% or more to be qualified.


6. Solder ball method


According to the relevant standards, select the solder balls of appropriate specifications and place them on the heating head to heat to the specified temperature


Place the test part (lead or pin) of the sample coated with flux horizontally and immerse it into the solder ball vertically at the specified speed


Record the time until the lead is completely wetted by the solder ball and completely covered


The solderability is measured by the length of this time. The time for the lead to be completely wetted by the solder ball is about 1s, and it is unqualified if it exceeds 2s.


7. Principle of Wetting Weighing Method


Suspend the sample of the component to be tested on the weighing rod of the sensitive scale; immerse the part to be tested in the molten solder (tin furnace) of constant temperature to the specified depth; the buoyancy and surface tension acting on the immersed sample are in the vertical direction The resultant force is measured by the sensor and converted into a signal, and is recorded as a force-time function curve by a high-speed characteristic curve recorder; this function curve is the same as the ideal wetting weighing curve (a fully wettable with the same properties and dimensions) The test samples) are compared to obtain the test results.


8. Coplanarity of component pins


The low coplanarity of the pins of surface mount components will affect the soldering performance and the good contact between the component pins and the PCB pads.


The standard tolerance of pin coplanarity for surface mount devices is 0.1mm


That is, the vertical distance between the highest sole of the pin and the plane formed by the soles of the lowest three pins is not more than 0.1mm.


9. Method for detecting the coplanarity of component pins


Place the components on a plane, use a measuring instrument to determine the magnitude of the deviation of the highest sole of the foot from this plane, place the components on the optical plane, use a microscope to measure the distance between the non-coplanar pins and the optical plane, use the vision The system (AOI) automatically detects.


10. The performance of components generally needs to be tested before the components are assembled, otherwise the cost of repair and rework will be very high.


Use general-purpose or special-purpose testing equipment to check the conformity between the actual performance parameters of the components and the nominal performance parameters.


11. PCB appearance defect detection


Alignment of the solder mask and the pad; whether the solder mask has abnormal conditions such as impurities, peeling, wrinkles, etc.; whether the reference mark meets the standard; whether the circuit conductor width (line width) and spacing meet the requirements; whether the multilayer board has peeling Layers and so on.


12. Solderability test of PCB


Test point: solderability test of pads and plated through holes


Test methods: edge dipping test, rotating dipping test, wave dipping test, solder bead test, etc. (IPC-S-804 standard, etc.)


13. Edge immersion test: used to test the solderability of surface conductors


The sample (edge) is immersed in the flux and then taken out, the excess flux is removed, and then immersed in the molten solder tank for a certain period of time, then taken out, and then visually inspected or evaluated with an optical instrument.




14. Rotating dipping test: solderability test of surface conductors and plated through holes


Fix the test sample on the rotating test arm, and the rotating arm rotates at a certain speed according to the adjusted track.


The surface impurity eliminator first removes the surface impurities of the solder through the molten solder tank,


The test sample will be immersed immediately afterwards (according to the standard immersion depth, stay on the molten solder surface for about 3s~5s),


After leaving the molten solder surface to cool down, perform visual inspection or evaluation with the aid of instruments.


15. PCB solder mask integrity test: There is a problem of solder mask coverage, which affects soldering. Under the impact of thermal stress generated by reflow soldering, the phenomenon of peeling and breaking from the PCB surface will occur.


The PCB used for SMA generally uses dry film solder mask or wet film solder mask


16. The solder paste is mixed with alloy solder powder, paste flux and some additives


17. In addition to the main contents such as viscosity, sag, solder ball, wettability, etc., visual contents such as the appearance and printing performance of the solder paste also need to be inspected.


18. Solder paste printing performance: During printing, it should be able to be transferred to the PCB smoothly and continuously through the template or screen without clogging the holes and the transfer is not smooth. The reasons for poor printing performance include insufficient co-printing agent, poor alloy powder shape, or particle distribution that does not meet the requirements, etc.


19. The typical viscosity of solder paste for SMT is 200Pa·s~800Pa·s


The main factors affecting the viscosity of solder paste: flux, alloy percentage, alloy powder particles


Shape and temperature


Solder paste viscosity test standard: IPC—SP一819


Solder paste viscosity detection method: rotary viscometer


20, solder paste thixotropic coefficient


The thixotropic coefficient characterizes the thixotropic properties of solder paste, and good solder paste has a higher thixotropic coefficient


21. Solder paste wettability: the ability of solder paste to wet and spread on the oxidized copper skin, which characterizes the degree of solder paste activity.


Commonly used test method: print solder paste with a diameter of 6.5mm and a thickness of 0.2mm on the copper skin, and the diameter should be enlarged by 20% to 30% after flow soldering, otherwise the wettability is considered poor.


22. Solder balls: In reflow soldering using solder paste, when the solder paste has quality problems such as moisture and oxidation, solder balls may be generated during soldering and scattered around the component pins. Cause problems: cause faults such as poor solder joints, or even short circuits.


23. Solder paste slump


Solder paste slump: After the solder paste is printed on the PCB and subjected to a certain high temperature, if the appearance of the edge shape is blurred and irregular, or even the interconnection of adjacent graphics appears in severe cases, it means that the solder paste has "collapsed" .


Causes problems: The sag phenomenon is one of the main causes of welding defects such as bridges and solder beads during reflow soldering.


Detection method: on the standard template, whether there is a bridge between the printed solder paste graphics to determine the collapse of the solder paste.


24. Method for detecting the percentage of solder paste alloy content: heating separation weighing method, the procedure is:


①Take 0.1g solder paste sample into the crucible;


②Heating crucible and solder paste;


③ Solidify the alloy and remove the flux residue;


④ Weigh the alloy weight, the percentage of alloy content = (alloy weight / solder paste weight) X 100%.


25. The oxidation of alloy powder is the main factor in the formation of solder balls and other welding defects. It is usually required that the surface oxide content of the alloy powder should be less than 0.15%.


26, solder paste adhesion


Requirements: The solder paste must have a certain adhesive force,


After printing, the components adhere to the required position,


No movement of components occurs during PCB transmission,


Generally, the solder paste is required to maintain sufficient adhesion within 8 hours after printing.


27. The role of flux: mainly to remove oxides on the solder metal surface, prevent re-oxidation of solder and soldering surface during soldering, reduce the surface tension of solder, enhance wettability, and accelerate heat transfer.


28. According to the traditional chemical composition classification


 There are four types of flux: rosin type (R), water soluble type (WS), synthetic activation type (SA) and low solid type (LS). The first three types generally contain 25% to 35% (weight) of non-volatile (solid) substances. Low-solid type contains less non-volatile substances (1% to 5% by weight).


Flux is classified by activity: it can be simply divided into three main types: low activity (L), medium activity (M) and high activity (H). Corrosive flux of fluxes marked "M" or "H".


Low solid flux (LSF) contains many active agents. The advantage of low solid flux is that it eliminates the need for cleaning after soldering.


29. The characteristics of no-clean flux: the residue is non-sticky. The flux and residues are non-corrosive and have sufficient activity to ensure acceptable welding quality.


30. Main inspection items: appearance inspection, foaming ability and solid content, expansion rate and relative wetting power, dryness of residue after welding, resistivity of aqueous solution, corrosion of copper mirror, chlorine content and acid value, insulation resistance, etc.


31. Flux expansion rate index: the expansion rate of inactive rosin flux (R) should not be less than 75%, the expansion rate of moderately active rosin flux (RMA) should not be less than 80%, and the expansion rate of fully activated rosin flux (RA) should not Less than 90%, the expansion rate of low solid no-clean flux should not be less than 80%.


32. No-cleaning flux: If rosin-type liquid flux or low-solid no-cleaning flux is used and it is not cleaned, the residue left after soldering is required to be dry (the residue is non-sticky), otherwise the adhesion of the flux residue may be Bring product pollution and even affect the electrical performance of the product.


33. The role of bonding agent:


For SMT, it should have suitable viscosity, low slump, fast curing, moderate bonding strength, high temperature resistance, good electrical properties, stable chemical properties, no peculiar smell, distinguishable color, stable storage and other properties.


The main items of adhesive testing: viscosity and thixotropy coefficient, bonding strength, spreading and slumping, curing time, electrical properties, appearance testing for deterioration, etc.


34. Bonding strength of the bonding agent: bonding the components to the PCB to ensure that they will not fall off due to vibration and thermal shock during the welding process.


35. Detection of cleaning agent and its cleaning effect


The main function of cleaning is to remove residual flux and residue on circuit components to prevent the circuit from being corroded.


The incoming cleaning agent inspection is carried out through methods such as appearance inspection, cleaning agent inspection (generally using gas chromatography (GC) method), and cleaning effect test.