[Coco] Sort of OT: Recapping

[Salvador Garcia]

 Thanks. Believe me, I considered doing just that, piggybacking the new cap on top of the old one's leads. The only thing I didn't like about this is that the cap's leads would be much longer and I didn't want them to act as antennas, especially with components working at giga Hertz speeds nearby.
Salvador


    On Saturday, November 28, 2020, 05:33:47 PM CST, Rick Ulland <rickulland1 at gmail.com> wrote:  
 
 I've got to admit, without tools I would probably just stick a new cap 
to the board with hotsnot and tack it to the remains of the old leads....



On 11/28/20 2:38 PM, Richard Lorbieski via Coco wrote:
> The problem is not with the copper thickness or amount of layers. Most 
> IBM retro motherboards (1990 era) are 6 or more layers.
>
> The three most common problems that occur with DIP multi-layer rework:
>
> 1) Failure to get the lead(s) out of the thru hole ground/power plane.
>
> 2) Old solder clogged inside the thru-hole of via.
>
> 3) Damage to the thru-hole plating (most catastrophic).
>
> I discourage the use of sharp objects or using brute force can lead to 
> damage the plating and thus cut the connection between internal and 
> surface pads of the PCB.
>
>
> On 11/28/2020 6:55 AM, Dave Philipsen wrote:
>>
>>> On Nov 27, 2020, at 11:09 PM, Richard Lorbieski via Coco 
>>> <coco at maltedmedia.com> wrote:
>>>
>>> Based on the photos, this is a multi layer IBM clone PCB with 
>>> possible internal ground and power planes.
>>>
>>> The problem is that the heat is being dispersed onto the ground (or 
>>> power) plane and not enough heat is being applied to the solder.
>>>
>>> This kind of rework requires more than a 70W soldering iron.
>>>
>>> You will need a rework station with a heat gun and possibly a 
>>> variable convection type oven.  A conventional oven could damage 
>>> components on the PCB or melt some of the plastic parts. You will 
>>> also need a soldering iron that is capable of reaching 900F and use 
>>> a proper solder tip. Most consumer soldering irons usually reach 
>>> 600F-650F.
>> You could even use a larger Weller “soldering gun” (you know, the one 
>> that actually looks like a gun!) to heat it up enough (for the 
>> weekend hobbyist who doesn’t do enough of this sort of work to 
>> justify the cost of a hot-air rework system or an oven). Keep a 
>> slight pull on the cap lead while heating the joint. As soon as the 
>> solder melts you pull the lead out and remove the heat of the gun. On 
>> radial lead parts like those in the photo you may have to alternate 
>> back and forth between the two leads a couple of times to “rock” it out.
>>
>> Practice on a junk board first until you feel comfortable with it.
>>
>>> I also recommend NOT to sharp knives or force the leads out of the 
>>> PCB. This could damage the internal connections on the PCB.
>> Although I would only recommend cutting the connection to the ground 
>> plane as a last resort, I would have to disagree with your comment on 
>> not using a sharp knife to cut the copper foil. The copper is 
>> relatively soft at a thickness of maybe 2 thousands of an inch. An 
>> Exacto or similar knife can cut through it fairly easily. However, 
>> the prepreg layers used to separate an internal copper layer of a 
>> 4-layer board are going to be probably more than 7 thousands of an 
>> inch thick and *much* tougher to cut through than the copper. When 
>> cutting through copper traces/planes it is very easy to see when 
>> you’ve gotten through the copper and reached the substrate. If you 
>> are fearful of cutting too deep, use less pressure on the knife and 
>> several successive cuts to get through the copper. I have never cut a 
>> trace deep enough on a multi-layer board to damage an internal layer.
>>
>>
>


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