Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.

Solder Balls in Boards


Solder Balls in Boards | 12 June, 2004

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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Sanjay Verma


Solder Balls in Boards | 19 June, 2004

Please bake PCB. And try other paste

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Solder Balls in Boards | 21 June, 2004

Dear Kanwaljit Singh,

my opinion is following: 1)solderpaste type is ok - usual (alpha-metals has this problem inside the paste paste- holder problem) - what do the operator when production change -clean well the stencil holes!! no visible rest on the mask -directly loaded? when stencil change he/she put the paste back to the not used paste? small paste remaining on the side of the holder (old paste) this defects the all quantity of paste inside the holder (the paste is not homogen more) IF ok then 2)pcb - boards during transportation/soreaging/loading in the screenprinter machine copper surface of the pcb is beein started oxidation or fat (from operator hands) is persent on the board 2 way to solve:- operator use esd handshoose - behind you they take of....board newer protected - use cleaning direct the board------if possible use water based material wich regeerate oxidation or cleaning fat or both. 3) sencil cleaning is fequent by you....ok try machine top leaning stencil(by Fuji GP64x series) when not possieble by hand at problematic aera and no cleaning material should be present after cleaning. is a question to try. whith this material is ok to try cleaning pcb IPA wet cleaningpaper is also ok but dangerous of health of operator, when not remowed paste is like bad solderpaste (cleaner goes inside the paste afther reflow 2nd phase is like a bomb internal the solderpaste.)

preaured air cleaned well the board. an extra conveyor part about 10cm and a mini zone....

to check open the owen during production.- let's see.

in future please send E-mail to

when you are located in EU I can help you directly on the line

send photo please to see the problem, but first try

Horv�th Szabolcs H-1675 Budapest Pf 83 Hungary

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