Is it really nessesary for PCB with BGA cases to be gold plated? Also I've heard that edge connectors (like PCIexpress etc.) must also be gold plated? Why is that? What are the consequences of not beeing gold plated?
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\$\begingroup\$ Oxidation springs to mind. \$\endgroup\$– Andy akaCommented Jul 1, 2020 at 11:51
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\$\begingroup\$ related: Surface finish for edge connector \$\endgroup\$– ArsenalCommented Jul 1, 2020 at 11:51
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1\$\begingroup\$ The usual English term for the case of an Integrated Circuit is "package" \$\endgroup\$– user20574Commented Jul 1, 2020 at 13:16
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\$\begingroup\$ This might be a myth, but in addition to the corrosion aspect, someone told me that gold-plated PCBs tend to withstand de-soldering and re-soldering better, making them more suitable for prototype boards where you expect to mess around with de-soldering. This also seems true in practice, from my experience. I could change SMD fine pitch components multiple times on gold-plated PCBs, where normally you'd lose some pads if you keep de-soldering components more than once. This might as well be because the gold-plated PCB was overall higher quality in the whole layer stack-up though? \$\endgroup\$– LundinCommented Jul 1, 2020 at 13:25
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4 Answers
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There are basicly two different sorts of gold plating. Soft gold and hard gold. Soft gold plating is very thin, its intention is to passivate the copper to provide long shelf life and because it is flat, it is suitable for fine pitch components like BGAs. One form of soft gold plating is called ENIG (eletroless nickle gold). It's a chemical process.
Hard gold plating is much thicker and applied using electroplating. This is used for edge connectors or other areas that will be subject to wear. Because more gold is used it's much more expensive.
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The gold plating is to prevent corrosion (which would increase electrical resistance) and to provide a better surface for the solder to bond to. The smoothness of gold plating is especially important for BGA components to ensure that all solder balls make contact with the board.
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\$\begingroup\$ Why is this important for BGA "cases"? And I thought solder didn't stick well to gold. The cheaper HASL is far better for that. \$\endgroup\$– pipeCommented Jul 1, 2020 at 11:53
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1\$\begingroup\$ never had any issue hand-soldering to golden contacts, both feel the same if you're soldering with soldering iron or air. If we're talking about soldering part of it only. Can't say much about how much better gold contacts are, but all the fancy connectors are usually gold. Like higher quality HDMI cables, connectors on motherboards etc. It has to mean something \$\endgroup\$– IlyaCommented Jul 1, 2020 at 11:56
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1\$\begingroup\$ HASL can't be used for fine pitch components as it's very difficult to ensure the components lie flat before reflow. Any slight bumps will mean some pads don't solder completely. Also solder (the S in HASL) corrodes very quickley so HASL has a short shelf life. \$\endgroup\$– Jay MCommented Jul 1, 2020 at 11:57
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\$\begingroup\$ @pipe BGA packages have an array of solder balls bonded to the pads on their bottom. The solder is reflowed using hot air to bond the package to the gold pads on the board. In my experience solder bonds very well to gold and with BGA the very smooth surface of the gold plating ensures that all solder balls in the array make contact. HASL on the other hand gives an fairly uneven surface. With HASL you may end up with some of the balls not being connected to their pads after hot air reflow. \$\endgroup\$ Commented Jul 1, 2020 at 12:03
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\$\begingroup\$ @JasonMorgan Yes, that is the answer to the question asked by OP. This answer at the point of my comment didn't touch any of that. \$\endgroup\$– pipeCommented Jul 1, 2020 at 13:29
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My understanding is it is not "necessary" for BGAs, but for large BGAs and large assembly runs ENIG provides a much smoother, oxidation proof surface so your yields are higher. You don't want to be reworking BGAs in a run making a thousand boards, or reworking big expensive BGAs. For one-off you could get away with it...I think.
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I don't see why BGA cases would specifically require gold plating. There are also other plating options. But gold plating improves the shelf life of PCBs, so you can store them longer before populating the board with components.
I also don't see why card edge connectors would specifically require gold plating, it might just be that it is the cheapest way to fullfill all electrical and mechanical requirements for a card-edge connections, even if other methods are available. There are several plating choises available when manufacturing a PCB, so the best for the application must be chosen, there is no single one best plating for every aspect.
The electrical conctacts must be within rated specifications, for example the resistance must be below a rated value after rated value of inserting and removing the card from connector 50 times. And it must do that under specific climate conditions that may cause corrosion unless taken care of. If a device does not fullfill e.g. the PCIe specifications, it can't be called a PCIe device then, and marketing it as such can be a violation of some contract regarding licencing the technology.
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2\$\begingroup\$ Simple answer. Lifespan. Gold does not corrode every other PCB passivation does, Properly stored gold plating lasts indefinately. Even silver or tin eventually corrode. You want a product to last long enough so you don't get it back before it's end of life and you don't want to present a poor reputation to customers. \$\endgroup\$– Jay MCommented Jul 1, 2020 at 12:16
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1\$\begingroup\$ BGA is just an example of a fine pitch component type that is easier to solder on a gold finish, there are many others and you don't have to use gold. Other flat passivations can be used, e.g. organic silver. But if you want to store bare boards the life of gold can't be beaten \$\endgroup\$– Jay MCommented Jul 1, 2020 at 12:19
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\$\begingroup\$ @JasonMorgan I understand that gold plating improves the storage life of the PCB blanks before they are mounted with components. But as the question was "is it really necessary", it is not really necessary. I will clarify my answer. \$\endgroup\$– JustmeCommented Jul 1, 2020 at 12:35