What are the pro's and cons of pasting ahead? The definition of pasting ahead for this question is the act of pasting 10 or 15 pc boards and then running then repeat. I do not like this philosophy. I'd prefer to paste, run, reflow then repeat. This eliminates the amount of time between paste application and the pc's entrance into reflow. The fresher the paste the better the solder joint. My dilema here is upper management wants the two lines to share one stencil printer thus saving the cost of purchasing a second printer. They propose to paste ahead, allow second runner to change stencils, paste their pc's, and so on. If shifts end approaches, they offer to store the pasted pc boards in a refridgerator overnight until population the next day. Help!! Does anyone out there find this to be a bit odd? Where could I find articles or documentation frowning upon pasting ahead? If I had something written by an "expert" maybe management would leave me alone and simply purchase a second stencil printer. Thanks for any suggestions you have.
I am sure if you contact your Paste Supplier they will give you all the advise you need. Better still contact a Printer Supplier and tell him to assist you in justifying the purchase of a new printer.(I am sure he would be more than happy!).
From my point of view you would be running totally inefficiently for a start not to mention the fact that any one of the Placement machines could breakdown at anytime.
I am sure if you carried out a cost analysis on Operator Changeover and Machine downtime waiting on boards along with Product Turnaround you will easily justify a new printer.( you can pick up printers quite cheaply).
If all that fails I would write a note on a letter and say "Aurevoir" and get the hell out of there.....
I don't know of any articles but you will see the following problems:
The flux will evaporate from the paste and your solder joint will look very grainy or will not reflow at all.
The paste will not have a good, tacky quality. Your pick and place or chipshooter will place small parts and they will move easily or not even get placed at all as they will stay with the nozzles instead of sticking to the paste deposit.
The results are hard to predict or put time limits on because there are too many variables such as stencil life of paste, humidity, etc.
Will they give you the opportunity to develop a DOE to look at quality vs. print life? It probably depends on the kind of placement equipment you have, but with our chipshooters we always had missing/misplaced parts if we populated dried out boards.
-The fresher the paste the better the solder joint. Agreed
- My dilema here is upper management wants the two lines to share one stencil printer thus saving the cost of purchasing a second printer. They propose to paste ahead, allow second runner to change stencils, paste their pc's, and so on. Is/are management process engineers savvy in the SMT process?? If so then they must know a little bit on failure analysis.
- If shifts end approaches, they offer to store the pasted pc boards in a refridgerator overnight until population the next day. Only if you can guarantee the refridge is plumbed with nitrogen (or inert atmosphere) as most refridges are not oxygen free that contributes to oxidation and they have a high degree of moisture..not good for reflow as can cause a popcorn type situation. Not to mention contamination left behind from yesterdays Chinese food. That General�s Chicken is hell on PCB�s
- Help!! Does anyone out there find this to be a bit odd? Very very Odd
- Where could I find articles or documentation frowning upon pasting ahead? If you are running standards then IPC J-STD-004 may help.
A Snippet from Loctite�s Link above: Use of solders with the correct rheological characteristic can eliminate such defects. Environmental variables such as temperature and humidity can affect the rheological characteristics of a solder paste. Solder pastes must be formulated to print satisfactorily across a wide range of environmental conditions, as maintaining a constant environment on the production floor is not always possible. Many electronic manufacturers have multiple locations with varying conditions in their production environment�further on... The ability of a solder paste to print acceptably after a delay in production is critical to reducing defects on the modern production line. A paste�s �abandon time� is defined as the maximum amount of time between two printing cycles that will provide acceptable results without reconditioning or kneading the solder paste or cleaning the stencil. Printer downtime is typically due to placement problems or operator breaks. Solder pastes with long abandon times reduce printer maintenance and solder defects due to paste drying, and improve subsequent processes in the line. Evaluating abandon time is very subjective as the definition of acceptable printing can vary greatly.
One printer 2 smt lines = No problem, but then you will need 2 ovens. It is even sometimes a more valuable cost savings depending on what production you are into.
At first the stencil printer with a divertion conveyor, then straight ahead into the 1 assy line, meanwhile you are setting up the next job in the second line or the secondary side of the product. A fast stencil printer will keep up with the production in 2 lines (assuming that you are not producing cell phones or simular). The drawback is that you need 2 ovens. We have this scenario with the DEK stencil printer and 2 smt-lines of NEXT machines and also a diverter at the end, feeding 2 ovens. Then we have a second line for prototype and NPI production only. You must do your math depending on how your production looks like. If your mangement goes for the key-numbers, meaning that the OEE and product changeover time is critical, maybe this is the best for that particulary solution.
But Never Ever store paste printed pcb:s into a fridge. That is totally outrages!
There are a numerius way of how to produce the boards throughout the smt-lines, but only one way to do it correct, according to the specified needs.
I suggest to review your board , do both boards have fine pitch QFP's or TSSOP's? . If yes, best approach is to finish board at a time to ensure best quality . You can refer to your solder paste TDS to justify why you choose this approach. If one of the board dont have fine pitch you can do manual print. You can use PCB break away as stopper . One operator do the squeege action, while other secure the alignment by holding the stencil frame ,old school but effective. If this is not allowed, you can design simple manual printer using simple backyard tools like C clamp and bearing. When I was in the manufacturing,I made one and proven to be effective .
We only allow 3 boards to be printed in front of the pick and place equipment. It is important to keep the abandon time to a minimum.
You did not mention the paste that you use. Do you use no-clean or water soluble? Do you use lead free?
Solder paste is hygroscopic which means it will absorb or expel moisture. Moisture can cause problems such as slumping, solder balls, splatter, and component popcorning. If your environment is too dry you paste will expel moisture (dry out). This will cause other soldering issues.