Hi Luigi, on top to Reuven suggestions, I would had also, paste volume could be the origin of the problem. This kind of defect is commonly called Solder Beading effect, often due to excessive paste volume under the chips. On IPC-7525, they suggest how to prevent or avoid this by designing dedicated stencil aperture shapes and the propose few examples like : Home Plate Aperture, Bow Tie Aperture, Oblog Aperture, etc. 3.2.3.5 Chip Components -Resistors and Capacitors Several aperture geometries are effective in reducing the occurrence of solder balls. All these designs are aimed at reducing excess solder paste trapped under the chip component. The most popular designs are shown in Figure 3-9 (same aperture design as the MELF diode) the C shaped aperture. This aperture design reduces the amount of solder paste under the Chip Component but maintains land coverage on the peripheral of the lands. Have you already in use stencils with aperture shape like above ? Suggestions like above have been experimented with success in plenty of cases. Best Regards Ciao Gabriele -----Messaggio originale----- Da: TechNet [mailto:[log in to unmask]] Per conto di Reuven Rokah Inviato: mercoledì 22 febbraio 2012 7.53 A: [log in to unmask] Oggetto: Re: [TN] Microballs Hi Luigi, You have to check: 1. Size of pads (too long under the components body). 2. Size of opening in stencil 3. P&P accuracy 4. Solder paste temperature (should be room temperature) 5. Oxidation of component / PCB [ads / solder paste particles Reuven On Wed, Feb 22, 2012 at 8:33 AM, <[log in to unmask]>wrote: > Hello everybody. > > Any idea about random presence of microballs (diameter 0.1 to 0.5 mm) > beneath some chips (R and C 0805, 1206,1210, 1812) ? > We use a SnPb process with a vapor phase oven. > We use same types of PCB, same process on same SMT line in a clean room > from years and the balls are randomly present from some months now. > They could be present in one lot of PCB and not the following; beneath a > component on one PCB and not on the same component on other PCB; we have > balls one day and not the day after; the balls disappear for weeks and > re-appear after; ... > I have checked different lots of solder paste, origin of PCB's, all > parameters of printing, P&P and the cleaning (co-solvent station). The > thermal profiles are same. > As it's hi-rel production I cannot change easely products and process. > I suspect a process parameter but which ? > I ask myself if there is not a ghost in the line .. :-) > > Best regards. > > CANTAGALLO Luigi > Technology Engineer > > > ______________________________________________________________________ > This email has been scanned by the Symantec Email Security.cloud service. > For more information please contact helpdesk at x2960 or [log in to unmask] > ______________________________________________________________________ > -- Best Regards, *Reuven Rokah* Mobile: 972-52-60-120-18 Tele-fax: 97239360688 <http://www.rokah-technologies.com/>[log in to unmask] [log in to unmask] www.rokah-technologies.com ** This e-mail message is intended for the recipient only and contains information which is CONFIDENTIAL and which may be proprietary of Rokah Technologies. If you have received this transmission in error, please inform me by e-mail, phone or fax, and then please delete all of the original files and all other copies exist. ______________________________________________________________________ This email has been scanned by the Symantec Email Security.cloud service. For more information please contact helpdesk at x2960 or [log in to unmask] ______________________________________________________________________ ______________________________________________________________________ This email has been scanned by the Symantec Email Security.cloud service. For more information please contact helpdesk at x2960 or [log in to unmask] ______________________________________________________________________