You state that the boards sometimes show warp right from the manufacturer,
that issue you should bring up with your board supplier.
For preventing warpage during reflow you might consider a center support for
the board (I assume the board is reflowed on some sort of edge conveyor and
is sagging?). My company manufactures reflow ovens that offer an Edge
Grabber conveyor (yes it actually holds the board edges to prevent the board
edges from curling) along with a center support conveyor. It has been proven
to decrease the amount of warpage during reflow considerably. I assume that
most reflow manufacturers offer a similar contraption to support the center
of the board, perhaps your reflow supplier would offer the center support as
a retrofit?
If you would like to discuss further you can contact me offline at the
following:
Brian Stumm
ETS, LLC
3939 North Freya Street
Spokane, Washington 99207
509-483-0900 (voice)
509-483-0331 (fax)
[log in to unmask] (e-mail)
www.eznet.com/~ets (internet)
>We have experienced the same problem as well, but we are short of board
>pallets. We cannot afford to use the board pallets to hold the boards
>while the boards are cooling. We end up using a cheaper method but it
>seems to work.
>
>After soldering, we put the board upside down onto some "standoffs" such
>that the board is "flat" (i.e. all edges are straight). We also use some
>weights on certain area to make sure the does not bow nor twist in
>cooling due to uneven contraction. This method is slightly messy and
>requires a bit of experiment about the height of the "standoffs" but it
>is cheap.
>
>Joseph Lam
>
>-----Original Message-----
>From: jim [mailto:[log in to unmask]]
>Sent: Tuesday, 12 January 1999 1:15
>To: [log in to unmask]
>Subject: Re: [TN] PWB Warping
>
>
>You may want to consider pallets to hold the board flat while it heats
>up
>over the way and as it cools down afterwards. The best solution is to
>fix it
>in the design, but if your already stuck with a design a stiffener or
>pallet
>will do the trick. Give me a buzz if you need stiffener or pallet info.
>Jim 972-494-1911
>-----Original Message-----
>From: Vascukynas, Brian <[log in to unmask]>
>To: [log in to unmask] <[log in to unmask]>
>Date: Friday, January 08, 1999 3:09 PM
>Subject: [TN] PWB Warping
>
>
>>Hello,
>>
>>We have been experiencing warping on certain PWB's from a specific
>>supplier.
>>I am curious on what the causes could be.
>>
>>In each instance, the PWB design layout was checked and found to be
>>well balanced. Also our SMT Reflow Profiles were looked at for each PWB
>>and found
>>to be typical. The boards sometimes exhibit a slight warp right out of
>>the MFG
>>packaging and will warp out of spec on the first run.
>>
>>1. We looked at dwell times, ramp up rate, actual temp, speed of PWB
>>going thru the
>>oven and all appears to be typical per each individual case. Is there
>>anything else we
>>could be checking for?
>>
>>2. Is moisture absorption a concern - could it cause warping ?
>>
>>3. Currently we do not bake PWB's prior to SMT Reflow - Is this
>>something we should
>>consider.
>>
>>Thanks in advance.
>>
>>Brian V.
>>
>>################################################################
>>TechNet E-Mail Forum provided as a free service by IPC using LISTSERV
>1.8c
>>################################################################
>>To subscribe/unsubscribe, send a message to [log in to unmask] with
>following
>text in the body:
>>To subscribe: SUBSCRIBE TechNet <your full name>
>>To unsubscribe: SIGNOFF TechNet
>>################################################################
>>Please visit IPC's web site (http://www.ipc.org) "On-Line Services"
>section
>for additional information.
>>For technical support contact Hugo Scaramuzza at [log in to unmask] or
>847-509-9700 ext.312
>>################################################################
>>
>
>################################################################
>TechNet E-Mail Forum provided as a free service by IPC using LISTSERV
>1.8c
>################################################################
>To subscribe/unsubscribe, send a message to [log in to unmask] with
>following text in the body:
>To subscribe: SUBSCRIBE TechNet <your full name>
>To unsubscribe: SIGNOFF TechNet
>################################################################
>Please visit IPC's web site (http://www.ipc.org) "On-Line Services"
>section for additional information.
>For technical support contact Hugo Scaramuzza at [log in to unmask] or
>847-509-9700 ext.312
>################################################################
>
>################################################################
>TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c
>################################################################
>To subscribe/unsubscribe, send a message to [log in to unmask] with following
text in the body:
>To subscribe: SUBSCRIBE TechNet <your full name>
>To unsubscribe: SIGNOFF TechNet
>################################################################
>Please visit IPC's web site (http://www.ipc.org) "On-Line Services" section
for additional information.
>For technical support contact Hugo Scaramuzza at [log in to unmask] or
847-509-9700 ext.312
>################################################################
>
>
>
################################################################
TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c
################################################################
To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body:
To subscribe: SUBSCRIBE TechNet <your full name>
To unsubscribe: SIGNOFF TechNet
################################################################
Please visit IPC's web site (http://www.ipc.org) "On-Line Services" section for additional information.
For technical support contact Hugo Scaramuzza at [log in to unmask] or 847-509-9700 ext.312
################################################################
|
