Reflow Profiling and Solder Voids

Void in the solder joint?is a problem and it needs to be addressed depending upon the criticality of the joint. Bottom Terminated component?e.g. BGA, is most affected by this problem. Problem has been more serious with advent of lead free solder and has been aggravated by?miniaturization of component as well as densely populated PCB with component having smallest and biggest in a very small real state of the PCB.

?What causes VOID?

Void is created in the reflow soldering, when the flux in the gaseous form gets trapped in the solder. In reality, the gaseous flus is supposed to come out of the joint but gets trapped and forms an area void of solder creating solder-void.

Void and Solder Paste

Solving a void problem through the chemistry of the paste has been the most worked area. Lot many chemistry and combination has been brought to solve the problem.But finding one solution to resolve the problem has been difficult.If one chemistry works best for some particular joint and combination of component , it does not work for many. Also, availability of paste as well as change of paste and its qualification poses real challenge to the user.

Void and Stencil

One of the possible solution to the void has been stencil opening for the component affected by void . Adjusting the paste volume as well as creating a space to allow the flux-gases to come out solves the problem of void.But due to geometry of the component and size of the component many a times this solution also does not work the best. Either it calls for many trials or it reduces the amount of solder at the desired joint. But in many cases it solves the problem to a great extent.

Void and reflow process

Vacuum reflow?is the ultimate solution to the void where soldering is done in a vacuum environment and flux gases are sucked out of the joint leaving void free solder joint. But availability and applicability of this is limited and hence does not provides universal solution of all the user. Similar solution is achieved by ultrasonic reflow also.

Void and Reflow profile

A typical reflow profile consist of four stages having preheating, soaking, reflow and cooling zone.

Each zone temperature characteristic as well as temperature requirements are different, accordingly their names are given. It is mandatory to control the temperature of the reflow oven very precisely in every zone??in order to protect the electronic component and most importantly formation of correct solder joint through accurate chemical & thermal equation.?

There are two main type of thermal profile one is called Ramp-Soak-Spike (RSS) and the other is called Ramp-to-Spike (RTS)

Ramp refers to the rate at which temperature changes over time and which is expressed in degree per second.In a typical case it ranges between 4 degree per second?and 2 degree per second.In the soak zone solder paste goes into phase change .In this zone most of the flux evaporates form the paste.A typical time range for this zone is 60 to 120 sec .In this zone if the heat is transferred at a higher rate there is a risk of Solder-spattering,Solder-balling,oxidation of pads & termination.Where as if the heat transer is done at a lower rate there is a risk of non-activation of flux giving rise to a risk of cold solder , incomplete solder as well as void .

The two types of thermal profile can be graphically represented as below:

Ramp-Soak-Spike permits the temperature change upto 4 degree per second where as in Ramp-to-spike it permits up-to one to two degree per second.

Indiaum Corporation refers about the Thermal Profiling as :

A thermal profile is an important record, process tool, and history of PCB assembly - an electrocardiogram (ECG) of your solder process. Profiling requires in-depth process knowledge and practice. Moreover, it is an art. Reflow soldering profiling warrants more discussion in detail…..Indium Corporation

To work on the void , a better understanding of the reflow profile is warranted.

A.???Soak zone calls for a balance of too slow soak and too fast soak. Too slow soak will invite the oxidation where as too fast soak will result into trapping of volatile gases inside the solder .When dealing with the problem of voids ,it is advisable that soak profile not to be used. And , if used its process window can be widen to reach the highest temperature as high as 200 degree centigrade which normaly gets restricted to 180 degree.

B.????To avoid the voiding ,?Ramp-to-Spike ( RTS) profile is used. Lower peak and ?higher peak temperature are the two options to be tried to find which gives the comparatively good result. Depending upon the PCB actual condition, either of these two can produce a better ?result.

C.????Time above Liquidous (TAL) is one of the key parameter to improve wetting as well as to reduce the probability of trapping of gas inside the solder. A longer TAL is advisable to fight with the problem of voiding. For example a 45-60 second TAL can be modified to 60-90 second but this may occasionally be found little bit out of specification referring to data sheet of the solder paste.

D.???Along with higher TAL , higher peak temperature is advisable for voiding issue.For example the peak temperature range 235-255 degree can be modified to 245-265 degree. This again may be found little bit out of specification referring to data sheet of the solder paste.

E.????Conveyor speed is one of the parameter which basically sets the limit to all the above parameter. While doing a parameter fine tunning , it is advisable , conveyor speed to be treated as a dependant parameter which will affect all the parameter simultaneously.

F.????There are many software available which can simulate the above parameter to get the expected profile instantly , which can be used to quickly arrive ?at a better decision.

Conclusion

a.????Key focus is on longer TAL

b.????Next key focus is on Higher Peak Temperature

c.????Solder paste specification and data sheet are guidelines

d.????Entire PCB to be taken into consideration while looking for an exact profile.

e.????Software can be used to balance all the parameter.