To Mod or Not to Mod? That is The Question

Many years ago, I was a product manager at a business-consumer electronics company developing some pretty leading edge display equipment. Prototyping back then was a long and painful process. A PC board might take a month or two to arrive from fabrication. Parts had to be sourced by digging through massive catalogs and then hoping that what you needed would be on the companies approved vendor list. The whole process was a bear.

Well, the soldering up part wasn't always so bad - unless you were the poor soul tasked with wire-wrapping or hand soldering the prototype.

Based on how difficult and expensive a board spin was back then, common practice was to just mod up the boards, even in production. Any given PCB might have a dozen or more cuts and mod wires. Those changes might not make it into the PCB for months. These days, though, you can get board fabbed Mod wireovernight, your parts delivered over night, and when you have all of those parts and PCBs, you can get them assembled overnight. I suspect that increase in speed is the major reason mod wires seem to be nearing extinction these days. (note that Screaming Circuits didn't build the board in this picture. It's from my personal collection)

It may not seem cheap to pay to have someone re-spin a board so speedily; especially when set next to hand soldering. But when compared to the cost of idle engineers waiting for the next rev, the cost of adding mods, the reduced reliability from having mods and the additional manufacturing time caused by modding a board; today's quick-turn parts, fab and assembly options can end up saving gobs of time and money in the long run.

Duane Benson
There are more wires in heaven and earth, Horatio, Than are soldered on your pc board.

How not to trick your BGA friends

Continuing with yesterday's theme, I have a couple examples that should have been fine, but due to issues at the board house, improper storage or contamination, ended up very much not fine.

What is wrongBehind door number one, we have an OSP finish that will make you very unhappy. That's "Organic Solderability Preservatives" in long hand. I've also heard it called "Organic Surface Preservative", but close enough. It is a nice flat surface which is good for BGAs. Years ago, it had a reputation for being poor quality, "cheap", but newer formulas seem to work pretty good in both leaded and lead free. In this case, the darker pads were likely contaminated in some way - either at the board fab house or subsequently in handling.

Siver migration problemNext is the worst example of surface degradation I've ever seen. Yes, it's an extreme outlier case, but this is where a silver board can go if it wasn't built with the best quality control, was stored too long, was exposed to polluted air or other contamination and had bad luck. This board probably has all of those issues, but any one alone can be problematic. Silver board especially should be stored in a cool dark place; preferably sealed in the original packaging.

Duane Benson
OSP can also mean Oregon State Patrol, but they don't care about BGAs. Just safe driving.

How not to treat your BGA friends

Over the years, most of what we see are good PC boards. But some standout in the other direction as examples of what not to do. Some didn't make it through the board house alive. Some were unknowingly rendered useless in layout and some were just held on to too long or not stored properly.

Large BGA via in padIn this first image, we see a guaranteed not to work example. Open vias in BGA pads will ruin your whole day. And you can't just cap them with solder mask either. For BGAs, the only two via solutions are to have them filled and plated over at the board house, or not be in the pads at all. Having a via in a BGA pad is like trying to cook scrambled eggs over a camp fire without a skillet. The eggs will in fact cook, but they'll be all mixed in with the fire and coals and stuff and you won't be able to eat them.

BGAB mask issuesThis next guaranteed not to work example shows a valiant attempt at keeping the vias out of the pads. But, as we used to say on the playground: "close only counts in horseshoes and hand grenades - and sometimes atom bombs." Here on the right, first, the mask registration is way off. That's not good but doesn't necessarily spell BGA death on its own. What will kill this assembly is the clear metal path between some of the pads and the vias. You need to have some soldermask blocking the metal path between the pad and the via. If you don't, it's almost as bad as putting the via in the pad. This board has a few places where there is a thin solder mask dam between the via and the pad. But, in the cases where there is no mask, the solder and solder ball will most likely migrate over to and down through the via.

Duane Benson
Close might also count with badgers.

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