Void and appearance observation

Voids have a large effect on the product life of products that have been used for a long time, in high temperature environments, in environments where temperature changes are drastic, and in environments where external stress is likely to be applied.
 
Especially in the case of in-vehicle equipment, it may be required to introduce a vacuum reflow furnace as a measure against voids, and recently there have been proposals for formic acid.
Solder manufacturers are proposing anti-void products, but at present there are advantages and disadvantages, and in any case it is hard to say that they are at a satisfactory level.
Void observation is generally X-ray observation, and it is observed with X-ray after mounting (after reflow), but at the actual mass production site, even if it can be selected by pass / fail judgment, there is no repair device like BGA, so it is corrected. Some parts cannot be used.
 
Also, for voids, if there is a problem even at one point (one place) of soldering, the board cannot be shipped, so it is theoretically necessary to observe all points.
 
There used to be a factory that had installed X-ray equipment in-line on all dozens of lines overseas, but it was not turned on.
The reason was that when the power of the X-ray device was turned on, there were many observation points, the tact slowed down, and the line stopped.
 
Even if various inspections are performed after mounting, only good or bad selection is possible.
Inspection is performed when manufacturing conditions are finalized before mass production, including X-ray observation.
After the start of mass production, it must be completed by sampling inspection.
 
In the current situation where the actual manufacturing engineering department has been lost, the following items are required before starting mass production.
 
1. Good / bad judgment by appearance observation
2. Good / bad judgment by X-ray observation
Verification of the relationship between
3.Verification of the relationship between 1 and 2
4. Associate other necessary observations / verifications
 
Void and appearance observation
 
Poor appearance implies the occurrence of voids.
Roughness of the flux residue and poor fillet gloss are due to deterioration of the flux and indicate poor fluidity of the solder.
Poor fluidity of the solder causes voids, so it is possible to simplify the inspection process after mass production by observing the part that seems to be a problem in the appearance observation with X-rays, analyzing the cause and taking countermeasures.
 
Case 1
 

 
In the future, X-rays will need to be at a level that allows analysis of the inside of through holes and the location of occurrence, rather than simply observing the presence or absence of voids.
 
Case 2
 

 
The position of the voids has a great effect on the joining quality.
 
1. Voids at the most dangerous locations
2. Occurrence position that does not affect much
 
Voids are problematic in terms of location and number (size). The shooting angle is important, not just the evaluation by area.
 

 
In split printing of solder, the flux residue collects in the center of the component and is not vaporized and is formed as a large and distorted void.
Small and round voids are due to residual gas, but highly heat-resistant flux residues have a significant effect on bond strength.
It is necessary to verify the printing method without mounting parts and obtain an appropriate mask opening shape.
Since this flux residue is scattered in vacuum reflow, cleaning is a necessary condition for high-density mounting boards.
 

 
The non-vaporized flux residue scatters during defoaming. If there are connectors around it, it may cause poor continuity, so cleaning is required.
It requires a gentle defoaming method and design consideration over time.
 
Introducing measures against voids in heat-generating parts
 
The following methods are required to take measures against voids.
 
・ Maintains solder fluidity by suppressing deterioration of flux
・ Reduces void generation rate by high-efficiency heating from base material to solder
・ The base material is heated by the floor heating effect of the lower heater.
・ Profile according to the characteristics of the flux
 

 

 
This is the result of demonstrating measures against voids by suppressing the deterioration of flux by turning off the upper heater of the preheat part.
 

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