Reflow process of chronic disease and solutions
     After assembly of electronic products from entering surface (SMT), large quantities of reflow process, the landmark legislation of the phenomenon of passive components to the circuit chip manufacturers to increase a lot of trouble . Quality and size chip devices continue to shrink, the application of high-temperature lead-free solder , landmark legislation has attracted people's attention. In this paper, to analyze the causes of the monument legislation , introduced landmark legislation to solve the basic idea .
Keywords: monument stand, heat capacity , temperature, nitrogen reflow, vapor phase reflow .
I. Overview
       After assembly of electronic products from entering the surface , bulk reflow process, the landmark legislation of the phenomenon of passive components to the circuit chip manufacturers to increase a lot of trouble .
Landmark legislation - the reflow process, some or all of passive devices is lifted , a small chip lead terminal device connected to the pad , and the other end is tall vertical lift lead , sometimes inclined sometimes device as upright as stone.
"Stone " such an analogy , it is very precise. After this landmark legislation requires welding defects rework operation, quality or cost due to the need to correct and be scrapped.
SMT manufacturing process at an early stage , usually monument legislation and vapor phase reflow ( condensation soldering ) linked together in a number of reasons , attributable to the rapid heating heating exigencies . With the recession in the gas phase reflow process , especially forced convection technology and advanced temperature control system , surface mount device soldering monument stand phenomenon almost disappeared .
However, the landmark legislation is not completely solved the problem away . Due to the quality and size of chip devices continue to shrink, the application of high-temperature lead-free solder , the monument stand again attracted attention . In the vapor phase reflow process nitrogen reflow system, new devices or PCB passive chip components smaller, do not want the monument was originally established phenomenon appeared again and resurgence .
Second, the study has its roots lie ?
        One of the reasons well known landmark legislation is the initial cause of the difference between the two solder wetting passive components . Uneven wet state is due to two different welding surface wettability and temperature. As the ideal state is a two -terminal devices simultaneously lead solder reflow form . At this time , the role of the both ends of the welding surface wetting force / surface tension effects cancel each other simultaneously , so as not occur project milestone .
If a lead terminal pads of the device quickly and wet reflux , acting on the lifting force of the solder joints formed with the lead terminal device . No melt solder the other end , the lead terminals by being wetted by the surface tension of the printed circuit board between the wet pad pulled fixed devices .
Third, what is the initial mechanism moist ?
Three important reasons for moist machine parameters ;
1 , the initial wetting time
2 , wetting forces
3 , complete wetting time
     As soon completely wet , will cause the occurrence of the monument site , because it is completely wet , and the device acting on the solder joint force is greatest.
One end of the device is assumed to achieve complete wetting speeds significantly faster than the other, it is possible to erect wetting force pulling device , which is due to the terminal through the paste if excessive force is exerted on the device side of the lead terminals and the top surface at right angles sake, the end of the device is not returning will be lifting off the pad , eventually leading to the monument stand phenomenon .
Fourth, the effects of the heat capacity of the welding.
Devices either directly welded end of the heat capacity of the monument legislation would affect production. Heat welding is caused by unequal capacity of the root causes of landmark legislation , the heat capacity of the smaller end of the first wet , so the gun first force is applied to the device , heat the lead end of the two passive devices may have a different capacity ; pad size tolerances, device lead end metallization tolerance , tolerance amount of solder paste printing , through-hole or the inner PCB layout and so on. Microscopic images of landmark legislation welding ends
4.1 PCB pad heat capacity
     The larger pad size , the greater the surface area of ​​the molten solder paste , the surface tension is also great. Changing pad size is large , the device vendors and device types to match the recommended pad sizes, but no manufacturing tolerances specified . Tolerance will pad heat capacity changes have a huge impact .
In addition, the pad dimensions and tolerances and component placement accuracy related. This , kind often the case , but not all , resulting in proportional relationship between the size of the pad / heat capacity and device specifications and established the monument . Recommended pad size and tolerance ;
4.2 heat capacity lead terminal device ,
     Associated with the device type and shape of the heat capacity of the welding process directly affects the heating rate and time. These tolerance values ​​indicate only normal , but relative, because with the miniaturization of devices , those with pads , metal and size parameters related to placement speed will become more important. Lead terminal device type and device form factor data shown in Figure 3 ; devices lead terminal type and shape of the device
The heat capacity of the paste 4.3
A small amount of solder paste reflow pad pads faster than the excess paste , regardless of what method , the pad must be deposited solder paste solder connection with the formation of a qualified match not excessive. More importantly, prior to reflow of the solder paste between the pads must be uniform . Three-dimensional image helps paste paste and engineers to detect heat capacity , so that it is under control .
Although a small amount of paste can be more rapid warming , but the mounting position of the device is actually also played the role in heating up the device mount alignment problems may also cause significant shift device leads side , this is bound to generate heat capacity is not Like , the result of two lead terminal temperature difference between the expansion
(Δt). To overcome this problem, paste must quickly melted in the fraction of a second .
Fifth, the temperature difference is as small as possible
Pad and the lead end surface oxidation and initial wet cleaning is fast , smaller surface tension , the larger the wetting force , quickly and completely wet . Assuming two lead terminals of the device is oxidized to the same extent , and some initial oxide surface wetting of the delay time , the initial wetting time is delayed , the more time the site at elevated temperature or the lead end of the pad to reduce the temperature difference between both ends (Δt) .
Empirically obtained ; smaller temperature difference (Δt), the time difference between the initial wetting is small , when the passive device is not the same at both ends wettability landmark legislation may arise because of the good weldability comparison will lead terminals faster achieve complete wetting .
The most common effects of wettability is that factor , for example ; ? Lead end metallization when the device is damaged , not properly coated or contaminated , which can reduce the surface area moist .
Li resistor monument visible microscopic image , not welded at the ends lifting show thinning coating reduces weldability ; lifting ends welded not show reduced solderability coating thinning ,
Sixth, the nitrogen reflow / vapor phase reflow
Is heated to reflux in the welding process , the nitrogen oxide can be prevented from re- soldering surface , help speed up the initial wetting . Vapor phase soldering process involves heating the welding process control . And refluxed in a nitrogen atmosphere similar to the reflux temperature in the vapor phase soldering process, the surface can be prevented from re- oxidation. These two processes, compared with the conventional reflow process , entering the reflow process , the oxidation of metal surfaces rarely occurs. Whereby a clean surface will soon be moist.
Not provide more rapid wetting time is reduced to reduce the temperature difference (Δt). Additional delay initial wetting , to reduce the temperature difference (Δt) is completely established to minimize the phenomenon of the necessary tablet . So nitrogen reflow and vapor phase soldering two processes can be achieved to reduce the incidence of landmark legislation .
Seven, temperature and surface state two factors
Table 1 lists produced two landmark legislation of factors: the surface of the printed circuit board and components including related factors, such as oxidative damage and weldability , coating. Temperature-related factors, such as temperature difference (Δt) and heat dissipation. As shown, the combination of these two factors affect the thermal stability of the alloy paste of choice must be considered.
The reason for the surface state of the temperature
Lead solder paste printing layer pad end
Processing dimensional tolerances
Pad device lead to mechanical damage or end surface
Or contamination. The inner layer of the multilayer PCB layout and routing through holes .
Device Mount
Mount force
Component placement position offset
Reflow Equipment
Nitrogen reflow oven during heating under reflux
Temperature gradient (ΔT) is too high
Paste
Thermal stability of paste flux / alloy
Type monument established causes
Eight paste Solutions
Landmark legislation to eliminate or minimize the occurrence of the monument legislation can be achieved by the choice of solder paste . First, the adhesive having a flux system thermal stability , the metal paste using the second material is particles of two different eutectic point ; 50% a melting point of 179 ℃, the other having a melting point of 183 ℃.
183 ℃ molten solder melting solids impede solder wetting force generated faster tilt angle effect. Another alloy solder paste pads 179 ℃ in a fraction of a second more time to wet , then restored the balance .
Klein established using the model to describe the phenomenon of the monument , a surface tension plays an important role . However, to eliminate the influence of the molten solder paste viscosity . Monument stand model - the role of the molten solder paste viscosity.
In the temperature range of 179 ℃ to 183 ℃ , the paste becomes suspended in the liquid mixture of Sn62 body such suspension significantly higher than the viscosity of the liquid completely , the higher viscosity of mechanical obstruction device inclined at an angle to balance the effect of the surface tension , the viscosity is an important parameter in this model should be attached .
Nine, complete solution
Monument established phenomenon generated by the following three basic principles of prevention ;
1 , control the temperature of the heating curve reflow process , minimizing the temperature difference (Δt).
2, the control PCB , components, component placement tolerances.
3, the control nitrogen reflow process oxygen component , should be less than 500ppm.
Li is a monument to prevent welding defects , as long as careful analysis of the reasons to be addressed to reduce its impact , and ultimately to achieve high productivity, low defect rate and lower rework costs .