Say you’ve got two parts you need to fasten. Glue sounds like a great option, right? It’s strong, fills gaps, and seems like the right fix. There’s just one problem: it’s an absolute nightmare to control. Glue can corrode materials, mar a product’s cosmetics, or weaken, causing the product to fall apart. Veteran engineers are nodding along knowingly but know that sometimes, they're forced to use glue anyway. In this article, we’ll discuss how to select, prepare and apply glue and detail best practices for the gluing process. We’ll also recommend our favorite and least-favorite glues.
Glue should be your last resort
If you need to fasten two materials, screws are your best bet. They are easy to design, cheap, repeatable, and reworkable. If not screws, there's tape, heat stakes, and snaps. And then there's glue. Glue is difficult to control -- but here's how to tackle it if you must.
1. Selection
Choose your glue based on all the potential scenarios the product may meet in the real world. For example, if your glue forms a water seal, ensure it’ll hold up not just in clean lab water, but also in soapy water, saltwater, or gasoline-contaminated water. All are terrible for glue. Also, know your glue’s properties. Some glues expand while curing, which poses a problem if you need components to be tight and rigid. Cyanoacrylate glue off-gasses, leaving a white haze on plastics. Glues vary in strength, viscosity, ability to fill gaps, working time, and cure time, so do your research.
2. Preparation
Follow the glue's instructions. Precisely. Contact the manufacturer directly if you’re unclear about the best way to process a glue. We’ve seen a team use a hot plate instead of an oven to prepare a heat-cure glue. Don’t take this shortcut — it affects the glue’s strength! Don't use expired glue or glue that hasn’t been stored correctly -- many types of glue need to be refrigerated.
Use the correct mixing nozzles and consider the ambient humidity and temperature. We’ve seen glues fail three months into a production run because the Shenzhen summer became very humid.
Also consider surface preparation -- typically, the bonding surfaces must be clean, dry, and rough. In particular, precede a gluing operation with adequate cleaning and drying steps, along with a primer or a plasma surface treatment if necessary.
3. Application
Adhere to the specified working time. If you take too much or too little time between dispensing the glue and pressing the parts together, the glue won’t reach its target strength. Read the instructions and time the application of the glue accordingly. Make sure to prime the nozzles well to ensure the mix is good. Some glues cost $700 for a small tube, but don’t be tempted to skimp on priming, setup, calibration runs and anything else needed to understand the true capability of your glue path and equipment. This upfront investment will save you a lot of pain on the backend.
Watch out for downtime and have a clear process to get your glue process up and running afterward. In the morning or after lunch, you may need to replace a nozzle or run a few test pieces to get the glue flowing again. Even letting the machine sit idle for 10 minutes between parts can affect dispense. To handle this, you can program a "keep warm" dispense every few minutes, even if it’s not going on parts.
4. Cure
Make your curing process repeatable. If you have a human squeezing parts together, the uncontrolled pressure will cause some glue joints to be too thin and others too thick. Instead, use a controlled fixture to repeatedly set the bond — even if it’s just a simple weight block.
Beware off-gassing. Remember, glue is a chemical mixture. Make sure yours is safe for the intended use. At one electronics company, an applied glue contained fluorine, which, when heated, created fluorine gas — which eats metals! The circuits in the product began to corrode, causing delays as engineers labored to figure out what was going wrong.
Don’t rush. Allow your glue to cure before you do mechanical or thermal reliability tests. Many curing profiles call for 24-72 hours to get to 80% strength.
Our recommendations
Designate a glue specialist. Make gluing someone’s problem to own. Keep records, characterize the process, and watch it like a hawk.
Create hard stops to control glue volume. Easier process control starts in the design phase! Include a ledge that sets the distance between the two glued surfaces. Glue strength is sensitive to that gap, so control it in your design.
Beware overflow and underfill. If you use glue, factor in potential variations during glue application. Design areas for glue overflow in case the volume dispensed is a little on the high side, and include some margin in the design so your product still holds together even if the glue area is smaller than expected. Consider automated optical inspection of glue after dispensing, but before assembly.
SuperX glue is great for electronics, image courtesy of Cemedine
Best glue: The best glue we’ve used is the SuperX 777 or 8008. It’s a single-part urethane glue that’s safe for electronics and doesn’t off-gas. It’s difficult to get in the United States, but easily available in Asia. It works like a charm — it won’t corrode a circuit board and doesn’t get so hard that it could rip off a part. It also doesn’t shrink when curing, which means no mechanical stresses.
Worst glue: Don’t use cyanoacrylate glue! Also known as Super Glue, it is terrible for most electronic products. It’s filled with corrosive chemicals that will off-gas, leaving a thin white haze on plastics, PCBs, and other components. Unless you really really know what you are doing, avoid cyanoacrylate glue.
There are times when glue is right, but if you can design it out of your product from the very beginning, you should. You’ll thank yourself later. If you must use glue, don’t underestimate it. That said, glue can be a reasonable consideration if you build a top-quality process around it.