Useful Information For Solder Paste Printing

 

In August 2009, I uploaded my blog regarding “Stencil Accuracy”, and now let’s talk about “The Coarseness in Stencil Opening” as I got the opportunity to do experiment on 0.4 QFP printing test.

 

(Provided) - Mask specifications are 0.18 x 1.47 mm(opening size), 0.13 mm (thickness), 5 μ (taper), electro polish, coarseness 5 μm (without coating), coarseness 4 μm (with coating), coat thickness 1 μm.

(Details are not able to be disclosed)

 

With Coating                                       Without Coating

 

           

 

 

Above photos show the comparison of PCBs printed after using stencil with coating and without coating at the 50th PCB after printing continuously without cleaning. The test result is still the same at every 10 pieces of PCBs. The difference of the paste shapes and the smearing is drastic in terms of with coating and without coating.

 

Without Coating

  1. Uneven paste height (upper photo)
  2. A lots of smearing (where arrows show)

 

Stencil without coating leads easily to unstable fillet, no solder, solder balls, and solder shorts because of the random paste volume. This causes uncertain consequences to and in AOI process.

 

The shape of fillet affects AOI pass-rate, largely. The question is how to improve and increase the accuracy of threshold setting towards higher separation of Real NG and False NG.

 

AOI fine tuning only in 1-2 pieces of PCBs does not mean the stability in a mass production. Indeed, it’s still considered as unstable. Does SPI work for this? SPI at average rank of popularity are unable to detect the defective as well, neither 3D-SPI nor 2D-SPI could not even judge each land paste volume deviation range.  

 

Good  Removability

Good removability leads to less smearing where it also contributes high output. Printing à Cleaning à Printing is a “changing point” process which shall followed. We could see there are a lot of NG cases occurred, it is because of neglect of changing point management, instead repeating the absurd counter-measure!

 

Less changing points or no changing points benefits the stability in a process.

 

Mask opening coating does not always refer to no-necessity of cleaning; instead, manual cleaning might not necessary.

 

Stencil maker must be checked its availability of coating and the difference of the opening size. 1 μm of thickness will not affect the test result.

 

1 μm of coating could put up with 1000 to 2000 times of printing without any wear and tear, the cost is still able to be covered with high output and quality.

 

:: Quoted by Mr. Berdie, 14-Dec-2009 ::