Solder Balls in Boards

Hi ! The main problem being faced currently is formation of solder balls - to be precise, these are micro solder beads generally found attached with bodies of SOTs, Chip Caps, Chip Resistors, between pins of SOICs (50 mil) - lying on PCB surface, sticked with the flux. The no of these beads vary from 5 to 10 per board to as high as 50-100 in another board, depending on the board types. Following are my observations - which are inputs for you.

a) Solder paste mostly used: Make: Koki, Product: SE4-M952K, Alloy: Sn63 Pb37, Flux: 9%, packed in 0.5 kg polythene bags of rectangular shape - directly loadable on he Paste printer M/c MINAMI # MK-878SV. Storage temp: 2-10 deg Celsius, Used after keeping for 6 hrs at ambient after taking out from Solder paste refrigerator. Expiry: 6 months. Paste height measurement: Every 2 hrs. using UV non-contact measurement M/C Z Check 600. Paste height / volume - seems to be ok. Prior to loading on Paste printer m/c, each packet of paste is stirred centrifugally on a m/c w/o opening the packet and then this poly pack s consumed within 2 hrs (max 12 hrs). Viscosity is not measured - but its look to be alright. Some shift print is observed - Not much. Stencil aperture reduction is done - generally upto10%. Aperture shape is also changed for some boards, as recommended by IPC to reduce solder balls formation due to paste printing related matters. Stencil cleaning is done automatically by the m/c and is programmed to do cleaning from bottom after every 5-10 boards.

b) Chip shooter & Fine Pitch placer All parametes seems to be under control. No misalignment, misplacement etc is observed so far.

c) Reflow (8 Zone, Top & bottom row of heaters, Non- Nitrogeneous atmosphere, chain conveyor) -No manual intervention for board movements till Post reflow is seen - PCB loading and unloading from/to SSRs is also automatic using loaders/unloaders. - Reflow profile is designed by Engg using recommendations of Solder paster mfr.and using most common industrial practices/guidelines. - On-line inspection of reflow board at Post reflow stage for inspection of solder fillets, misalignment, tomb stoning, less/no solder etc.

Reflow is done as per the following instruction sheets: Ramp up < 2.5 C/sec Pre-heat 140 deg C - 160 deg C; Time 60-120 sec Over 200 deg C; Time 30-60 sec Peak temp 210-230 deg cm/min. Conveyor speed: 68.0 c

Profile is taken using four themocouples on every 3 rd day and /OR on every product change Delta Temp. as observed on today's profile is as follows: Pre-heat: 3.9 deg C Peak temp: 1.7 deg C

Now that you have gone thru these inputs. Thanks for your patience. Pl advice what more specifically to observe. Why the solder balls/beads are still being formed?? Waiting for your response. Thanks & BR, Kanwaljit

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