John,
The first thing I thougt of was the temperature of the board or
carrier. The room temperature may be 70dF, the parts may not.
I had the experience already in 1985. We installed an air conditioned
room with T= 22dC (72dF).
We started in february with no problems until the sun took strength.
We had wide glass windows on 3 sides. And slump all over on sunny
days.
We could not produce on sunny days before we had installed sunshades
outside the building.
Lesson: the board to be printed has to meet the temperature
requirements to prevent slump.
Second thought: did you execute the slump test.
Peer Langeveld
Consultant Soft Soldering Processes
5502 VH 8
The Netherlands
2009/2/3 John Anselmo <[log in to unmask]>:
> Randy,
>
> Yes. On one board we build, the bridging is (90% of the time) on the same end and same side of the connector.
> On other boards it is not.
>
> John
>
> -----Original Message-----
> From: Randy Morrison [mailto:[log in to unmask]]
> Sent: Monday, February 02, 2009 3:38 PM
> To: TechNet E-Mail Forum; John Anselmo
> Subject: RE: [TN] Bridging on SMD Connector
>
>
> Is there any pattern to the bridging (ends vs middle, etc.)?
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of John Anselmo
> Sent: Monday, February 02, 2009 2:25 PM
> To: [log in to unmask]
> Subject: Re: [TN] Bridging on SMD Connector
>
> The stencil is wiped after ever 2nd pass.
> I have not had a chance to measure the stencil (I don't have the proper
> tool).
> The flex is 8 to 10 mils thick and it has a 20 mil stiffener (I was
> wrong when I said it was 10 mils).
> The panel is approximately 9" x 9". The board itself is a 2.4" x 2.4"
> "L" shaped flex cable.
> So far the paste is all from the same lot.
>
> Your last question brings up an interesting point.
> We only have a limited number of carriers, so the operators sometimes
> cool them just until they can be handled safely, them load up another
> panel.
> Could this be causing some of the slump issues? We do get bridging all
> the time, but the degree of bridging does sometimes change.
>
> John
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]]On Behalf Of Guy Ramsey
> Sent: Monday, February 02, 2009 1:56 PM
> To: [log in to unmask]
> Subject: Re: [TN] Bridging on SMD Connector
>
>
> Hey man,
>
> Are you getting a clean wipe?
> Is it possible that your stencil does not meet spec?
> How thick is the flex, how deep is the recess?
> How small is the board?
> The carrier could shim the stencil off the board.
>
> Is the paste all from the same lot? I have never had problems with 562
> but anything is possible.
> Is the carrier hot when you print?
>
>
>
>
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of John Anselmo
> Sent: Monday, February 02, 2009 2:50 PM
> To: [log in to unmask]
> Subject: [TN] Bridging on SMD Connector
>
> Hi all,
>
> I am having some difficulties with bridging on an SMD connector and need
> some help.
> I've searched the archives for similar problems but have come up empty
> so far. Below are the details:
>
> We are placing two different JMD connectors, one 60 pin and one 80 pin.
> Stencil: 5 mil stencil and the apertures are 1:1 with the pads (.020"
> pitch, .012 pad width)
> Paste: Kester R562 (use a new jar every day)
> Parts: Flex boards with .010" stiffeners supported by carriers (the flex
> panels are placed in a Durastone carrier that is recessed the thickness
> of the panel).
> Printer: MPM model AP25
>
> We are measuring the paste parameters after printing with the
> CyberOptics 2000. The data shows lots of variation in paste volume and
> thickness.
> I am not sure how accurate the data is since the Cyberoptics does not
> verify correct alignment of the panel before it takes the measurement,
> but I do agree we have variation.
> In general, it shows the paste thickness to be .007" to .009".
>
> We see slumping of the paste sometimes before placement, mostly after
> placement.
> I've changed the parameters of the printer (pressure, print speed, the
> snap off distance to ensure we are getting on-contact printing).
> I've also had the operator rotate the stencil 90 deg to see if there was
> a difference in how we applied the paste (parallel or perpendicular to
> the length of the aperture).
> We are using metal squeegees on the printer.
> All of the above changes have resulted in no noticeable reduction in the
> number of bridges.
>
> We are currently running a flex panel with 12 parts per array and we get
> 50% to 100% bridging per panel. Now you see why I am pulling my hair
> out.
>
> The room parameters (throughout the day) are 70 to 74 deg F and 25% to
> 30% RH
>
> My questions are:
> 1) What else can I look at that effects bridging?
> 2) Can the reflow (hot air forced) oven be causing some of the problem?
> 3) What should the paste thickness be with a 5 mil stencil?
> 4) Should we go to a .001" reduction in the aperture on the stencil?
> .002"
> reduction?
> 5) Is our panel carrier causing the problem?
> 6) Will rubber squeegees make a difference?
>
> Frustrated in Montana,
> John
>
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