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How long can you leave paste - update

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Quick question for anyone - With this whole pasting and wait... - Apr 02, 2007 by chrissieneale  

#48811

How long can you leave paste - update | 2 April, 2007

Quick question for anyone - With this whole pasting and waiting thing - i've found out that the board is pasted, it sits around for a bit, then the components get placed on it. Then i thought it went into the oven BUT, it doesn't.

It sits for a few hours (sometimes overnight) and waits for someone to place the handfit components on it THEN it goes into the oven.

How long can components sit in paste before bad things start to happen?? I'm in a non controlled environment and when it's hot outside, it's hot inside (ditto cold). Lets say worst case senerio board gets pasted at 9am, sits for 5 hours, has components placed (ranging from 0402s to 1256 BGAs) by machine, sits around for 19 hours, has handfit components placed then goes in the oven. Finally.

Hands up who thinks i've got quality problems?

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#48812

How long can you leave paste - update | 2 April, 2007

Yes, Hand Up here. I would think you would have a quality problem. I am assuming you're doing this process in some sort of batch mode.

I would hope that you can process your boards within a few minutes after components being placed. Things that I would worry about is fulx starting some sort of contamination on your leads and BGA's. My process state that the board must be placed in the oven within 30 minutes of printing. But, I am running an inline process that allows me that kind of control.

Worse case, I would say not to wait no more than an hour or two.

Really, the only truly correct way to figure this out in your eviourment is to run a DoE and see what happens. I would start out by printing a series of PCB's and process them at 45 minutes apart.

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#48813

How long can you leave paste - update | 2 April, 2007

Please tell us you are not making any function critical items like pace makers or aircraft navigation equipment.

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#48815

How long can you leave paste - update | 2 April, 2007

Hand on heart nothing that will kill anyone. :) Promise.

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#48816

How long can you leave paste - update | 2 April, 2007

Hi Chris,

Yes, my hand is up. But the problem is WHY you are stock piling printed boards. Is it a single printer used to feed various lines? I would think that since you are hand placing parts, that this printer would be able to keep up. Obviously if your solder joints meet your needs, there isn't a over-driving problem. If you are looking for a cheap means to just get better, you may want to look and a manual or hand solder platform. They do work well and could probably keep up with your hand placing of parts. The same operator that places parts can manually screen print the board first. Just an idea for you to kick around.

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#48818

How long can you leave paste - update | 2 April, 2007

I totally agree, if there is a bottleneck in the process anywhere then i think it's got to be hand fit, so that shoudl be driving the demand for the rest of the process. I need to time/measure things to prove it though.

Great Monday discovery! :)

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#48819

How long can you leave paste - update | 2 April, 2007

Hand up here too. While most pastes have some flexibility and you may get by without too many problems, I don't think any of them are designed with long wait times in mind.

What I'm confused about is what your operator is doing when boards ARE running on the machine. Does he/she have time to do the hand-adds while machine processing (what we do) or is the hand add process the bottleneck?

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#48820

How long can you leave paste - update | 2 April, 2007

Change operator to OPERATORS (there's 3 per shift, 2 shifts) and then you can join me in confused bubble...

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#48823

How long can you leave paste - update | 2 April, 2007

How many lines? Or do you not really have lines?

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#48825

How long can you leave paste - update | 2 April, 2007

One. Consisting of:

1 stencil printer (approx 10yrs old) 4 placement machines (2 obsolete, 2 fairly new) 1 oven (fairly old)

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#48827

How long can you leave paste - update | 2 April, 2007

Are you running different baords/product at the same time? Why are they buffering boards after screen print?

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#48836

How long can you leave paste - update | 3 April, 2007

Apparently because the machines are old and prone to break down they do all the printing for the day (maybe 6/7 different board types) up front.

This doesn't make any sense to me - if the machine is going to break down, the machine is going to break down, it doesn't matter if you have your boards pasted up or not. Plus this then see the boards stacking up and waiting for the hand placement which only one person can do.

I'm still trying to figure it all out.

Anyone know what happens to a BGA if it sits in paste for up to 12 hours?

C

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#48842

How long can you leave paste - update | 3 April, 2007

OK - going to set up some measurements so we can see how big this problem is. I've got the following so far...

Number of boards pasted per employee per shift Number of hand placed parts per employee per shift Number of people per shift Time pasting machine is utilised in a shift Time pick and place machines are utilised in a shift Time oven is utilised in a shift Time spent hand placing parts in a shift Machine down time per shift Number of hand placements per board Number of different boards per shift Number of boards in each batch Complexity of board Number of boards cleaned and repasted Time from pasting board to reflow oven Time board takes from start (picking up the job) to finish (on to next stage) Time taken to set up first board Time taken to place first component after pasting Time taken to place last component after pasting

... any more for any more?

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Haris

#48843

How long can you leave paste - update | 3 April, 2007

Hello, simply downkload your solderpaste specification from the internet and look, u will find your desired answer. bye

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#48846

How long can you leave paste - update | 3 April, 2007

> Hello, simply downkload your solderpaste > specification from the internet and look, u will > find your desired answer. bye

She might want accuracy not certainty.

Or to put another way in theory there is no difference between theory and practise, in practise there is.

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#48852

How long can you leave paste - update | 3 April, 2007

Personally I wouldn't count anything but minutes. You need to know how long it takes to perform each task in order to balance your line.

What I'm gathering is that you've got one printer feeding 4 machines, all running different jobs or maybe a pair tag-teaming a job here and there, right? So, you print up one job then start printing another, regardless of how long the boards will sit before being populated.

Of course this is all speculation, but it sounds like you need another printer, big time, if you really need all that placement capacity to get the work out the door.

On the other hand, if you've split every job up into four programs, it must be exciting when you have a new job come in. :)

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#48862

How long can you leave paste - update | 4 April, 2007

The two machines in the middle are really really old m/c's so we try as much as possible to not to use them (apart from as very expensive conveyor belts). They run a fixed set up as although we have a bunch of different products they all tend to use the same parts. Give or take. So the only set up time there is with the board is reloading a feeder if the parts have run out. All four machines do the same job - and the printer is actually about a third quicker than the placement on most jobs (the two machines in the middle have the old style vibrating feeder that throws chips upsidedown - anyone remember them?). Then they all stack up at hand fit before going through reflow.

Personally i can't really understand it. It must be one of those therory vs. practice things.

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Haris

#48909

How long can you leave paste - update | 6 April, 2007

Basically its spec. gives surety regarding its chemical and physical properties. if he doesnt follow the spec then may be he may find good physical soldering appearance but chemically it may goes down through Electrical or Environmental testing and if he is working on Class 3 products then ...u can think of that.

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#48936

How long can you leave paste - update | 9 April, 2007

> Basically its spec. gives surety regarding its > chemical and physical properties. if he doesnt > follow the spec then may be he may find good > physical soldering appearance but chemically it > may goes down through Electrical or Environmental > testing and if he is working on Class 3 products > then ...u can think of that.

Nothing gives phsyical surety, only liability surety.

I think if you checked on how the paste manufacturers come up with their specs you would disillusioned.

As much as I hate being cynical I usually find I"m not cynical enough.

Sometimes I wish I could have blind faith in people I"ve never met. (the people who wrote the specs.)

Stay within specs, just don't expect them to be 100% accurate.

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#48942

How long can you leave paste - update | 10 April, 2007

Until it drys out but why would any reasonable person want that.

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Reflow Oven