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Application guidelines of Alpha Water Soluble solder paste (WS type)
Following guiderlines were recommended when applying Alpha water soluble pastes such as WS601, WS605, WS609 etc.
Stainless steel or nickel-plated brass may be used . Stainless steel stencil are more popular in usage owing to its durability. For critical application in obtaining accurate printing definition, especially in fine pitch, laser fabricated stencil is preferable to chemical milled stencil.
Stencil thickness varies with the stencil aperture configuration or the lead pitch. Suggested stencil thickness for applying Alpha paste is as follows:
|
Lead pitch (inch) |
Aperture width (inch) |
Stencil thickness(inch) |
|
0.050 (1.27mm) |
0.023 (0.584mm) |
0.010~0.012(0.25~0.30mm) |
|
0.025 (0.635mm) |
0.011 (0.279mm) |
0.006~0.008(0.15~0.20mm) |
|
0.020 (0.508mm) |
0.009 (0.229mm) |
0.005~0.007(0.13~0.18mm) |
|
0.015 (0.381mm) |
0.007 (0.178mm) |
0.003~0.005(0.08~0.13mm) |
In case of solder bridging defects/open solder defects due to inappropriate amount of paste applied, the stencil thickness or aperture may be varied to tackle these defects.
|
Registration for the printer(board & stencil) |
0.0005”(0.013mm) in precision is preferred especially for fine pitch printing application |
|
Printing mode |
Print in one direction |
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Squeegee hardness |
80~90 durometer for single thickness stencil, 70~80 durometer for step stencil |
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Squeegee pressure |
Avoid excess pressure. Use the squeegee pressure such that paste is just cleared from the stencil surface after the printing stroke. |
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Snap-off |
0.005”~0.050”(0.127~1.27mm) |
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Squeegee shape |
Diamond or rectangular blade |
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Squeegee speed |
0.1~1.0” per second. If possible, use the slower speed which imparts more consistent print height and the the height will be closer to stencil thickness. |
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Production environment |
30~60%RH, 20~26° C |
Alpha WS paste have a initial tack force of approximately 2g/mm square. The tack force will gradually decrease after deposition. It is therefore best to place components just after printing when the paste surface is at its wettest form. Small components can be placed up to 4 hours from printing. Excessive pressure should be avoided because such action squeezes paste off of pads causing solder balls. This problem is especially noticeable with chip components. The humidity of production environment should be below 60% R.H. in order to avoid moisture absorption by the paste deposition.
A variety of equipment including infrared, vapor phase, conductive belt or hot air can be used to reflow Alpha WS paste. Reflow by IR/hot air is the most popular technique.
A representative IR profile is given below as starting point to estabilsh one’s own profile:
1). Cure – 100 ~ 140 ° C for 2 minute, Ramp rate – 1 ~ 3 ° C/sec.
2). Pre-heat – 140 ~ 160 ° C for 1 minute, Ramp rate – 1 ~ 2 ° C/sec.
3). Reflow – 210 ~ 220 ° C for 20 ~ 40 sec., Ramp rate – 0.5 ~ 1 ° C/sec.
Total heating dwell time may be 5 ~ 7 minute depending on thermal inertial & component sensitivity.
Below are guidelines on these method removing the flux residues:
a). Cleaning Solder Stencil
During printing, the bottom side of stencil should be dry wiped. Avoid mixing the cleaning solvent with the paste. For top side cleaning, excess paste should be removed from the stencil or other parts (e.g. rubber squeegee) by alcohol based Alpha thinner.
b). Cleaning Mis-screened Panels
Removal of the unwanted paste from the panel can be done by squeegee, by wiped cleaning with Alpha thinner, etc. If thorough cleaning is required, the panel can then be cleaned aqueously by hydrocleaner.
c). Cleaning Flux Residue After Reflow
Warm water (45 ~ 70 ° C) without saponifier is effective in removing the residues. A rinse with D.I. water is recommended. Cleaning of flux should be done within 24 hours after reflow.
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