PCB hole size and track adhesion

[ruffrecords]

For several years I have used a couple of PCB suppliers. Both are based in the UK but manufacture in China. The quality has always been first class. In the last couple of years I have tried going direct to overseas suppliers, particularly for prototype quantities, simply because they are very cheap. I have used Iteadstudio and Seeedstudio. All my boards are through hole - no SMT.

Many of my boards use 1mm pins. They have knurled sides which are a tight push fit into the specified sized holes. This has always worked as expected with my UK based Chinese suppliers. However, both Iteadstudio and Seedstudio boards using these pins appear to have oversized holes. The pins just drop in - they are no longer a push fit. This is easy to work round but a bit annoying. Makes me wonder whether they skimp on the through hole plating.

I also had a problem with some electrolytic capacitors - the leads of some seemed to big for the PCB hole. Never a problem with my regular supplies. Worked round this by making the footprint hole slightly larger but it makes me wonder if they run their drills for too long so they eventually make under size holes.

Both these issues I can live with for prototypes, but recently I was building  a new prototype board from Seedstudio. I had to remove and replace a leaded capacitor. It came out with ease but when I went to replace it I noticed the pad on the component side had come away with the capacitor. I have never had this happen on a new board before. Makes me wonder, yet again, if they skimp on their through hole plating. There is no good work around for this type of limitation. I have contacted Seedstudio to complain about this.

So for a recent new design I searched for a UK prototype manufacturer and found one that was not too expensive. I had just one PCB made. As I was building/testing it I needed to change a couple of capacitors. I de-soldered them with the solder sucker and replaced them Unexpectedly, that part of the circuit stopped working. Sure enough another pad had come off and I had to add a wire to fix it. So it looks like the pad problem is not just  a Chinese issue -  is it just me being too ham fisted? I seem to remember from years ago modifying prototypes like mad but rarely if ever having tracks come adrift.

Am I expecting too much from today's PCBs?

Cheers

Ian

 

[Gareth Connor]

Hello Ian,

I use both UK and Chinese suppliers for PCBs, with probably upwards of 1000 boards a year being purchased, spread over 30 to 40 designs. Boards are split about 60% SMD and 40% through hole, with all SMD boards having some through-hole parts. Most boards are double sided, with around 20% being multilayer.

So far I have not had the problems you note on a first re-work of a board. I have had pad-lift on some 2nd reworks, but it is a rare problem.

I specify finished hole sizes as being after plating. Hole size as being after plating is traditionally the assumed standard for fabrication, but pointedly specifying it does force the requirement onto the PCB supplier.

Copper weight could be a factor on the pad adhesion. The usual spec I issue for copper weight is 1oz for signal PCBs and 2oz for PSUs, power amps or other boards where high-currents are likely to be present.

Hope this helps.

Gareth.

 

[ruffrecords]

Hi Gareth,

Thanks for the input. Your experience parallels my own from many years ago. It is only recently that I had seen pad lifting problems. I tend to put ground planes on the solder side and interconnect on the component side of my double sided. I wondered if this could be part of my problem.  A typical leaded component only has a pad on the solder side where you apply the iron for de-soldering. The track is on the other side so I wonder if it is not getting enough heat. The other thing I should do is clip out the component I am removing and just remove its leads rather then trying to remove the whole component.

Cheers

Ian

 

[Gareth Connor]

Hi Ian,

Thanks for the additional info.

Considering that we have both been in electronics for "a year or two" and the pad-lift problem is a recent one, it could well be a materials issue - I noted previously that pad-lift has been found to sometimes happen on 2nd re-works. Thinking about this a bit deeper, there certainly was less pad-lift years ago....... the tools are the same  (except for consumables such as soldering iron tips and solder-sucker nozzles), the methods are the same, but the boards are not. I'll ask a colleague what his experiences are - on a day-to-day basis, he has more to do with older boards than I do, so could provide another viewpoint.

One of the desoldering problems I have had with groundplanes is when there is a component pad in the middle of a plane of copper. The heat from the iron just dissipates and does nothing useful to melt solder. Using heat-relief pads solves the problem, however, the way the heat relief is applied during design needs careful thought with regard to copper weight, gaps between the pad and the plane, the number of connections to the plane, the width of the connections, and whether the component leg connects to another ground-plane on the other side of the board (or to inner layer groundplanes).  Phew, quite a number of variables!

Your component removal method of clipping and removing component legs one-by-one is a definite winner for double-sided re-work. It is my preferred method for components where I can get at the legs.

Cheers,

Gareth.

 

[JohnRoberts]

Over the last decade or so most of my PCBs come from China... I don't do much rework on production boards (that's the reason for prototypes). I did experience one run of prototype boards where the foil adhesion was noticeably inferior. I tested by intentionally pulling a PCB trace from one spare proto board and compared that to other boards I had laying around. Not scientific but definitive IMO.

I mentioned it to the vendor who did not admit a problem. I suspect they encountered a bad batch of copper clad... stuff happens.  I purchased from that same vendor again and my subsequent prototype PCBs were fine.

JR

 

[peters]

I'm not sure what they call it but many PCB designs will have some openings in the ground plane near the plated through holes.
This relieves some of the heat sinking of the ground plane so you don't need as much heat when soldering/desoldering on the top side.

 

[Matador]

It's called a 'thermal relief'.