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SMT electronics assembly manufacturing forum.


Help Bottom Side Components

Kevin Facinelli

#4449

Help Bottom Side Components | 12 April, 2000

I am interested in any techniques that may be utilized for bottom side soldering of Chips and Resistors on circuit cards populated on both sides containing through hole parts.

Our current process is :

Stencil solder paste Place a glue dot Place Parts Convection Reflow

We our stenciling on solder paste to eliminate opens. Has anyone had luck eliminating glue from their process? How?

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#4450

Re: Help Bottom Side Components | 12 April, 2000

Kevin: You're correct. Loose the glue, unless you're waving second side or have real heavy components. Check the archives for component weights, references, and what not.

I believe that most people (let's see a show of hands kids) are using two paste/reflow cycles, which is the trade that you're making by using the glue. This is because most people don't have luxury of a dispenser.

Aww, now I've got myself wondering if there isn't more to life than throughput, which you certainly give-up to dispense. Yeh, I wouldn't dispense based on the information you've given us, but then again if Kevin is the first person to sandbag information. I'd use the dispensor for times when it gave an advantage, like quick turn and small lot size work.

Dave F

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#4451

Re: Help Bottom Side Components | 12 April, 2000

Kevin: I'm back.

Essentially what you're doing is: trading the cost of a stencil against the throughput you give-up by dispensing.

Interesting Dave F

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Micah Newcomb

#4452

Re: Help Bottom Side Components | 12 April, 2000

Is this board going across wave? If not, you are pretty safe to lose the glue process. If it does go across wave you can probably lose the solderpaste and replace your solder stencil with a glue stencil, much faster and efficient than dispensing. It helps if your wave has a "chip wave" to eliminate opens. if not look at the orientation of parts on the bottom side and try to adjust accordingly. If the layout is not suitable for wave try a 45 degree angle across the wave. My 2cents.

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#4453

Re: Help Bottom Side Components | 12 April, 2000

Hi Kevin!!! I have worked with both sides boards in 3 differents ways:

a)Glue-Solder paste- Manual assembly- Wave soldering For the bottom side we placed glue dot in the board then we placed parts and finally the convection reflow at 150�C each zone. After that for top side we printed solder paste, mounted components and reflow. Finally we mounted thru-hole components by manual assembly and after it wave solder process. Then we changed from placed glue dots to printed the glue in the board with stencil. With this process I usually got missing components at bottom side after wave soldering.

b) Double reflow (with thru-hole) If you have chips from 0204 to SOIC 8 in bottom side you can use this process, first you will need a stencil with thru-hole apertures fot the top side.

Run bottom as follows: Stencil solder paste Place SMD components Convection reflow

Then run top side as follows: Stencil solder paste Place SMD components Before oven place thru-hole components (automatic or manually) Convection reflow

This process gave me some problems with the thru-hole components as solder balls, solder insuficiency. So we changed to the next process...

c)Double reflow - Wave selective soldering Here you won�t use the stencil with the thru-hole apertures but you will need selective soldering pallets. In this process you will do the same as shown in the number two without place thru-hole components before the reflow oven, those components are going to be mounted by manual assembly. In this station you will placed the board on the selective soldering pallet and this will cover all the chips showing to the wave only the leads of the thru-hole components. This process is more expensive than the others because you have to use the pallets and wave solder process but it works better for my products.

I hope this helps you. Regars.

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Kevin Facinelli

#4454

Re:Help!! Couple more questions | 13 April, 2000

Couple more questions:

The problem we have is that on the bottom side we are using a bunch of resistor networks. These have been very difficult to solder through the wave.

The products:

Top Side: 4-6 BGA 2/3 QFP 500 components

Bottom Side: 400 Res/Cap 0603 100 Resistor networks

Our Process:

1st pass- "Bottom" Stencil Paste Glue dot Place Reflow

2nd Pass- "Top" Stencil Paste Place Reflow

3rd Pass- Insert Thru hole Mask Resistor Networks "Wonder Mask" wave solder

The designs I am working with have not been designed for a selective solder palette. I have many of the resistor networks close to through hole leads. We use this process because we have had poor results during wavesolder with the resistor networks. I am wondering is it possible to use a special paste/conductive adhesive that could be stenciled on during the first pass creating good component adhesion necessary during third pass wavesolder along with a conductive connection to the board.

Special Paste? Special Adhesive?

Any Ideas?

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pascal MATHIEU

#4455

Re: Re:Help!! Couple more questions | 14 April, 2000

Hi Kevin , we use frequently double reflow + selective soldering after , and try to avoid the glue process as much as possible because that 's not very reliable process in comparison to reflow process . We begin now to work on PTH components soldered in reflow mode but in this case we have to take in account the increase of the price of the PTH component (the plastic part must witdstand the high reflow temperature ) so ... for your purpose , if you have few pin through hole components and because of design it's not possible to have a selective soldering , perhaps you can have a manual soldering operation ? i have a question : how do you do with the "wonder Mask" to prevent the flux of the wave flowing inside the hidden area ? or perhaps you don't mind about that ; let me know .

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